Add telemetry metrics API with structured tags#108
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Adds metrics_api = telemetry as a new option alongside the existing folsom and exometer backends. Instead of encoding the pool name in a flat binary metric name, the telemetry backend calls the user module as execute([pooler, MetricName], Measurements, Metadata) where Metadata always carries pool_name and type, and enriched counters also carry reason (e.g. max_lifetime, crashed, at_capacity). Metric names use clean base atoms without the legacy _count/_rate suffix (free, in_use, take, etc.) and proper types: gauge for point-in-time counts (previously histogram in folsom) and counter for take_rate (previously a folsom meter). Folsom and exometer backends are unchanged in behaviour via compatibility delegate clauses. Also adds pooler:metrics/0 which returns a descriptor list for all emitted telemetry metrics, intended for pre-declaring prometheus.erl metrics at startup. The return value is a compile-time literal so there is no per-call allocation. Fixes a pre-existing bug in pooler_sup:pool_child_spec/1 where per-pool metrics_mod/metrics_api config was silently overwritten by the application-level defaults. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
The comment was left in pooler.erl (truncated) when terminate_pid/2 was extracted to pooler_starter.erl in d7a525f. Restored and moved to sit above the function it describes. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Adds metrics_api = telemetry as a new option alongside the existing folsom and exometer backends. Instead of encoding the pool name in a flat binary metric name, the telemetry backend calls the user module as execute([pooler, MetricName], Measurements, Metadata) where Metadata always carries pool_name and type, and enriched counters also carry reason (e.g. max_lifetime, crashed, at_capacity).
Metric names use clean base atoms without the legacy _count/_rate suffix (free, in_use, take, etc.) and proper types: gauge for point-in-time counts (previously histogram in folsom) and counter for take_rate (previously a folsom meter). Folsom and exometer backends are unchanged in behaviour via compatibility delegate clauses.
Also adds pooler:metrics/0 which returns a descriptor list for all emitted telemetry metrics, intended for pre-declaring prometheus.erl metrics at startup. The return value is a compile-time literal so there is no per-call allocation.
Fixes a pre-existing bug in pooler_sup:pool_child_spec/1 where per-pool metrics_mod/metrics_api config was silently overwritten by the application-level defaults.