perf: cache the key used for OTEL traces and metrics

The HeaderInterceptor creates a key consisting of the database name and method name
that is used for OpenTelemetry attributes and metrics. The number of unique keys is
low. However, the key is constructed from the DatabaseName and method name every time,
which leads to a lot of string creation:
1. The DatabaseName.toString() method is called every time. This constructs a new string.
2. The result of DatabaseName.toString() is concatenated with the methodName to create yet
   another string.

Instead of creating the key every time, we can cache the key values without doing the string
creation and concatenation every time.

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/HeaderInterceptor.java

@@ -87,6 +87,8 @@ class HeaderInterceptor implements ClientInterceptor {
       CacheBuilder.newBuilder().maximumSize(1000).build();
   private final Cache<String, Map<String, String>> builtInAttributesCache =
       CacheBuilder.newBuilder().maximumSize(1000).build();
+  private final Cache<DatabaseName, Cache<String, String>> keyCache =
+      CacheBuilder.newBuilder().maximumSize(1000).build();
 
   // Get the global singleton Tagger object.
   private static final Tagger TAGGER = Tags.getTagger();
@@ -116,7 +118,7 @@ public void start(Listener<RespT> responseListener, Metadata headers) {
         try {
           Span span = Span.current();
           DatabaseName databaseName = extractDatabaseName(headers);
-          String key = databaseName + method.getFullMethodName();
+          String key = extractKey(databaseName, method.getFullMethodName());
           TagContext tagContext = getTagContext(key, method.getFullMethodName(), databaseName);
           Attributes attributes =
               getMetricAttributes(key, method.getFullMethodName(), databaseName);
@@ -201,6 +203,13 @@ private Map<String, Long> parseServerTimingHeader(String serverTiming) {
     return serverTimingMetrics;
   }
 
+  private String extractKey(DatabaseName databaseName, String methodName)
+      throws ExecutionException {
+    Cache<String, String> keys =
+        keyCache.get(databaseName, () -> CacheBuilder.newBuilder().maximumSize(1000).build());
+    return keys.get(methodName, () -> databaseName + methodName);
+  }
+
   private DatabaseName extractDatabaseName(Metadata headers) throws ExecutionException {
     String googleResourcePrefix = headers.get(GOOGLE_CLOUD_RESOURCE_PREFIX_KEY);
     if (googleResourcePrefix != null) {