Document summarizer Prometheus metrics and fix stale lag_epochs help

Add a chaind_summarizer_* section to docs/prometheus.md.  All seven
summarizer metrics were undocumented (six pre-date this branch — the
gap dates to whenever chaind_summarizer_* was introduced — plus the
new lag_epochs from this branch).  Documenting the full set in one
place catches the doc up with reality and gives operators a complete
reference for the new alert.

Fix the Help string on summarizerLagEpochs: it claimed the gauge
measures lag against "the finality target", which was true under the
original formula in commit 066bb38 but no longer matches the cursor-
diff formula introduced in the previous commit.  Replace with "lags
its direct upstream cursor" so /metrics, the docs, and the ADR all
agree on the gauge's contract.

Author: AntiD2ta

docs/prometheus.md

@@ -24,6 +24,13 @@ Operations metrics provide information about numbers of operations performed.  T
   - `chaind_finalizer_latest_epoch` latest epoch processed by the finalizer module this run of chaind
   - `chaind_proposerduties_epochs_processed` number of epochs processed by the proposer duties module this run of chaind
   - `chaind_proposerduties_latest_epoch` latest epoch processed by the proposer duties module this run of chaind
+  - `chaind_summarizer_epochs_processed_total` number of epochs processed by the summarizer module this run of chaind
+  - `chaind_summarizer_latest_epoch` latest epoch processed by the summarizer module this run of chaind
+  - `chaind_summarizer_days_processed_total` number of days processed by the daily-rollup submodule of the summarizer module this run of chaind
+  - `chaind_summarizer_latest_day` latest day (Unix timestamp of midnight UTC) processed by the daily-rollup submodule of the summarizer module this run of chaind
+  - `chaind_summarizer_balance_prune_ts` Unix timestamp of the last validator-balance prune run by the summarizer module
+  - `chaind_summarizer_epoch_prune_ts` Unix timestamp of the last epoch-summary prune run by the summarizer module
+  - `chaind_summarizer_lag_epochs{pipeline="epoch|block|validator"}` number of epochs by which each summarizer pipeline lags its direct upstream cursor: `pipeline=epoch` is the gap between `validators.latest_balances_epoch` and the summarizer's epoch cursor (i.e. how far the upstream balances pipeline is ahead of summarization); `pipeline=block` and `pipeline=validator` are the gaps between the summarizer's epoch cursor and its own block / validator sub-pipeline cursors. Values are clamped at zero — disabled pipelines (e.g. `summarizer.blocks.enable=false`) produce no series for that label
   - `chaind_validators_epochs_processed` number of epochs processed by the validators module this run of chaind
   - `chaind_validators_latest_epoch` latest epoch processed by the validators module this run of chaind
   - `chaind_validators_balances_epochs_processed` number of epochs processed by the balances submodule of the validators module this run of chaind

services/summarizer/standard/metrics.go

@@ -124,7 +124,7 @@ func registerPrometheusMetrics() error {
 	summarizerLagEpochs = prometheus.NewGaugeVec(prometheus.GaugeOpts{
 		Namespace: metricsNamespace,
 		Name:      "lag_epochs",
-		Help:      "Number of epochs by which each summarizer pipeline lags the finality target",
+		Help:      "Number of epochs by which each summarizer pipeline lags its direct upstream cursor",
 	}, []string{"pipeline"})
 	if err := prometheus.Register(summarizerLagEpochs); err != nil {
 		return errors.Wrap(err, "failed to register lag_epochs")