forkable: add WithFinalizedBlockNumMetric option#61
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Reports the LIB number of the block that just became head, on the live metrics path next to the existing head block number and drift metrics. Paired with the head block number metric, it lets dashboards show how far behind finality an app's head is. The option takes a minimal Uint64Metric interface rather than a concrete type, so consumers (firehose-core) can own the metric definition without bstream depending on them. It is a separate option instead of a change to WithMetrics so existing callers keep compiling.
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Why
Firehose apps expose
head_block_number{app}but nothing telling how far behind finality that head is. firehose-core is adding afinalized_block_number{app}metric so dashboards can compute head↔finality drift in blocks.The relayer's head metrics are set inside the forkable's live-metrics path, so the finalized number has to be set there too — computing it elsewhere (a ticker, a per-source handler) would report head and finalized from different observation points.
What
forkable.WithFinalizedBlockNumMetric(m Uint64Metric)option. On the live path only, it reportsblk.LibNumof the block that just became head, next to the existing head block number / time drift / relative drift metrics.Uint64Metricis a minimalinterface{ SetUint64(uint64) }so consumers own their metric definition and bstream does not have to depend on them.WithMetrics, so existing callers keep compiling.Tests
forkable/metrics_test.gocovers the value being set fromLibNumon the live path, and not being set when live metrics are off.