feat(metrics): add block transaction count histogram for empty block monitoring

Replace the dedicated tron:block_empty_total counter with a more comprehensive
tron:block_transaction_count histogram that tracks the distribution of
transaction counts per block.

Changes:
- Add overloaded init() method in MetricsHistogram to support custom buckets
- Add BLOCK_TRANSACTION_COUNT histogram with buckets [0, 10, 50, 100, 200, 500, 1000, 2000, 5000, 10000]
- Record transaction count for all blocks (including empty blocks with txCount=0)
- Empty blocks can be queried via bucket le=0.0
- Remove unused BLOCK_EMPTY counter

This provides richer insights for network analysis while still supporting
empty block monitoring via histogram bucket queries.

Closes #6590

Author: warku123

common/src/main/java/org/tron/common/prometheus/MetricKeys.java

@@ -14,7 +14,6 @@ public static class Counter {
     public static final String TXS = "tron:txs";
     public static final String MINER = "tron:miner";
     public static final String BLOCK_FORK = "tron:block_fork";
-    public static final String BLOCK_EMPTY = "tron:block_empty";
     public static final String SR_SET_CHANGE = "tron:sr_set_change";
     public static final String P2P_ERROR = "tron:p2p_error";
     public static final String P2P_DISCONNECT = "tron:p2p_disconnect";
@@ -64,6 +63,7 @@ public static class Histogram {
     public static final String MESSAGE_PROCESS_LATENCY = "tron:message_process_latency_seconds";
     public static final String BLOCK_FETCH_LATENCY = "tron:block_fetch_latency_seconds";
     public static final String BLOCK_RECEIVE_DELAY = "tron:block_receive_delay_seconds";
+    public static final String BLOCK_TRANSACTION_COUNT = "tron:block_transaction_count";
 
     private Histogram() {
       throw new IllegalStateException("Histogram");

common/src/main/java/org/tron/common/prometheus/MetricsCounter.java

@@ -14,7 +14,6 @@ class MetricsCounter {
     init(MetricKeys.Counter.TXS, "tron  txs  info .", "type", "detail");
     init(MetricKeys.Counter.MINER, "tron  miner info .", "miner", "type");
     init(MetricKeys.Counter.BLOCK_FORK, "tron  block fork info .", "type");
-    init(MetricKeys.Counter.BLOCK_EMPTY, "tron  empty block count .", "miner");
     init(MetricKeys.Counter.SR_SET_CHANGE, "tron sr set change .", "action", "witness");
     init(MetricKeys.Counter.P2P_ERROR, "tron p2p error  info .", "type");
     init(MetricKeys.Counter.P2P_DISCONNECT, "tron p2p disconnect .", "type");

common/src/main/java/org/tron/common/prometheus/MetricsHistogram.java

@@ -48,6 +48,11 @@ public class MetricsHistogram {
     init(MetricKeys.Histogram.BLOCK_FETCH_LATENCY, "fetch block latency.");
     init(MetricKeys.Histogram.BLOCK_RECEIVE_DELAY,
         "receive block delay time, receiveTime - blockTime.");
+
+    init(MetricKeys.Histogram.BLOCK_TRANSACTION_COUNT,
+        "Distribution of transaction counts per block.",
+        new double[]{0, 10, 50, 100, 200, 500, 1000, 2000, 5000, 10000},
+        "miner");
   }
 
   private MetricsHistogram() {
@@ -62,6 +67,17 @@ private static void init(String name, String help, String... labels) {
         .register());
   }
 
+  private static void init(String name, String help, double[] buckets, String... labels) {
+    Histogram.Builder builder = Histogram.build()
+        .name(name)
+        .help(help)
+        .labelNames(labels);
+    if (buckets != null && buckets.length > 0) {
+      builder.buckets(buckets);
+    }
+    container.put(name, builder.register());
+  }
+
   static Histogram.Timer startTimer(String key, String... labels) {
     if (Metrics.enabled()) {
       Histogram histogram = container.get(key);

framework/src/main/java/org/tron/core/metrics/blockchain/BlockChainMetricManager.java

@@ -174,11 +174,11 @@ public void applyBlock(BlockCapsule block) {
       MetricsUtil.meterMark(MetricsKey.BLOCKCHAIN_TPS, block.getTransactions().size());
       Metrics.counterInc(MetricKeys.Counter.TXS, block.getTransactions().size(),
           MetricLabels.Counter.TXS_SUCCESS, MetricLabels.Counter.TXS_SUCCESS);
-    } else {
-      // Empty block
-      Metrics.counterInc(MetricKeys.Counter.BLOCK_EMPTY, 1,
-          StringUtil.encode58Check(address));
     }
+    // Record transaction count distribution for all blocks (including empty blocks)
+    int txCount = block.getTransactions().size();
+    Metrics.histogramObserve(MetricKeys.Histogram.BLOCK_TRANSACTION_COUNT, txCount,
+        StringUtil.encode58Check(address));
 
     // SR set change detection
     long nextMaintenanceTime = dbManager.getDynamicPropertiesStore().getNextMaintenanceTime();

framework/src/test/java/org/tron/core/metrics/prometheus/PrometheusApiServiceTest.java

@@ -84,11 +84,13 @@ protected void check(byte[] address, Map<ByteString, String> witnessAndAccount)
     Assert.assertEquals(pushBlock.intValue(), blocks + 1);
 
     String minerBase58 = StringUtil.encode58Check(address);
+    // Query histogram bucket le="0.0" for empty blocks
     Double emptyBlock = CollectorRegistry.defaultRegistry.getSampleValue(
-        "tron:block_empty_total", new String[] {"miner"}, new String[] {minerBase58});
-
-    Assert.assertNotNull(emptyBlock);
-    Assert.assertEquals(emptyBlock.intValue(), 1);
+        "tron:block_transaction_count_bucket",
+        new String[] {"miner", "le"}, new String[] {minerBase58, "0.0"});
+    
+    Assert.assertNotNull("Empty block bucket should exist for miner: " + minerBase58, emptyBlock);
+    Assert.assertEquals("Should have 1 empty block", 1, emptyBlock.intValue());
 
     // Check SR_REMOVE for initial address (removed when addTestWitnessAndAccount() is called)
     Double srRemoveCount = CollectorRegistry.defaultRegistry.getSampleValue(
@@ -99,7 +101,8 @@ protected void check(byte[] address, Map<ByteString, String> witnessAndAccount)
     Assert.assertNotNull(srRemoveCount);
     Assert.assertEquals(1, srRemoveCount.intValue());
 
-    // Check SR_ADD and empty blocks for each new witness in witnessAndAccount (excluding initial address)
+    // Check SR_ADD and empty blocks for each new witness in witnessAndAccount
+    // (excluding initial address)
     ByteString addressByteString = ByteString.copyFrom(address);
     double totalNewWitnessEmptyBlocks = 0;
     for (ByteString witnessAddress : witnessAndAccount.keySet()) {
@@ -114,13 +117,17 @@ protected void check(byte[] address, Map<ByteString, String> witnessAndAccount)
           new String[] {"action", "witness"},
           new String[] {MetricLabels.Counter.SR_ADD, witnessBase58}
       );
-      Assert.assertNotNull("SR_ADD should be recorded for witness: " + witnessBase58, srAddCount);
-      Assert.assertEquals("Each new witness should have 1 SR_ADD record", 1, srAddCount.intValue());
+      Assert.assertNotNull("SR_ADD should be recorded for witness: " + witnessBase58,
+          srAddCount);
+      Assert.assertEquals("Each new witness should have 1 SR_ADD record", 1,
+          srAddCount.intValue());
 
-      // Collect empty blocks count
+      // Collect empty blocks count from histogram bucket
       Double witnessEmptyBlock = CollectorRegistry.defaultRegistry.getSampleValue(
-          "tron:block_empty_total", new String[] {"miner"}, new String[] {witnessBase58});
-      Assert.assertNotNull(witnessEmptyBlock);
+          "tron:block_transaction_count_bucket",
+          new String[] {"miner", "le"}, new String[] {witnessBase58, "0.0"});
+      Assert.assertNotNull("Empty block bucket should exist for witness: " + witnessBase58,
+          witnessEmptyBlock);
       totalNewWitnessEmptyBlocks += witnessEmptyBlock;
     }
     Assert.assertEquals(blocks, (int)totalNewWitnessEmptyBlocks);
@@ -176,14 +183,17 @@ public void testMetric() throws Exception {
     Map<ByteString, String> witnessAndAccount = addTestWitnessAndAccount();
     witnessAndAccount.put(ByteString.copyFrom(address), key);
 
-    // Explicitly update WitnessScheduleStore to remove initial address, triggering SR_REMOVE metric
+    // Explicitly update WitnessScheduleStore to remove initial address,
+    // triggering SR_REMOVE metric
     List<ByteString> newActiveWitnesses = new ArrayList<>(witnessAndAccount.keySet());
     newActiveWitnesses.remove(ByteString.copyFrom(address));
     chainBaseManager.getWitnessScheduleStore().saveActiveWitnesses(newActiveWitnesses);
 
     // Update nextMaintenanceTime to trigger SR set change detection
-    long nextMaintenanceTime = chainBaseManager.getDynamicPropertiesStore().getNextMaintenanceTime();
-    chainBaseManager.getDynamicPropertiesStore().updateNextMaintenanceTime(nextMaintenanceTime + 3600_000L);
+    long nextMaintenanceTime = 
+        chainBaseManager.getDynamicPropertiesStore().getNextMaintenanceTime();
+    chainBaseManager.getDynamicPropertiesStore().updateNextMaintenanceTime(
+        nextMaintenanceTime + 3600_000L);
 
     for (int i = 0; i < blocks; i++) {
       generateBlock(witnessAndAccount);