fix(telemetry): bound Prometheus distribution buffer (fork pin)

[robacourt]

Summary

Fixes unbounded ETS growth in the Prometheus metrics reporter. When ELECTRIC_PROMETHEUS_PORT is set but the /metrics endpoint is scraped infrequently — or never, as on OpenTelemetry-only deployments — telemetry_metrics_prometheus_core buffers one ETS row per distribution observation and only drains it on scrape. The per-transaction receive_lag distribution dominates, so the prometheus_metrics_dist table grows without bound. A customer hit an 8 GB+ ETS table and eventual OOM this way.

This pins telemetry_metrics_prometheus_core to a fork that bounds the buffer by aggregating automatically — on a size threshold (default 10k buffered samples) and a time fallback (default 60s) — in addition to on scrape, and serializes aggregation in the registry process (also fixing a pre-existing overlapping-scrape race).

Upstream PR: beam-telemetry/telemetry_metrics_prometheus_core#77

Why a git dep is safe here

The telemetry deps (the electric_telemetry path dep and its transitive telemetry_metrics_prometheus_core) are gated behind MIX_TARGET=application in sync-service/mix.exs (telemetry_deps(_) -> [] otherwise). The Hex-published electric package is built with the default target, so this dependency is absent from the published dep tree — the git pin cannot affect mix hex.publish. It only affects the standalone sync-service / Docker / CI build, where mix deps.get fetches git deps fine (git is installed in the builder image). electric-telemetry itself is not published to Hex.

Changes

• packages/electric-telemetry/mix.exs — pin to the fork branch, with a comment to revert to ~> 1.2 once upstream lands on Hex.
• packages/electric-telemetry/mix.lock + packages/sync-service/mix.lock — locked to the fork commit (only that one entry changes).
• Changeset (@core/sync-service patch).

Verified electric-telemetry compiles against the fork. Revert this pin to the Hex release after #77 merges and ships.

🤖 Generated with Claude Code

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
4704	1	4703	66

View the top 2 failed test(s) by shortest run time

Elixir.Electric.ShapeCacheTest::test get_or_create_shape_handle/2 against real db crashes when initial snapshot query fails to return data quickly enough

Stack Traces | 0s run time

30) test get_or_create_shape_handle/2 against real db crashes when initial snapshot query fails to return data quickly enough (Electric.ShapeCacheTest)
     test/electric/shape_cache_test.exs:508
     ** (EXIT from #PID<0.9368.0>) killed

Elixir.Electric.ShapeCacheTest::test start_consumer_for_handle/2 starts a consumer plus dependencies

Stack Traces | 0.0374s run time

1) test start_consumer_for_handle/2 starts a consumer plus dependencies (Electric.ShapeCacheTest)
     test/electric/shape_cache_test.exs:1559
     ** (FunctionClauseError) no function clause matching in Support.TestUtils.wait_until/2

     The following arguments were given to Support.TestUtils.wait_until/2:

         # 1
         1000

         # 2
         #Function<75.111972680/0 in Electric.ShapeCacheTest."test start_consumer_for_handle/2 starts a consumer plus dependencies"/1>

     code: assert wait_until(1000, fn ->
     stacktrace:
       (electric 1.7.4) test/support/test_utils.ex:146: Support.TestUtils.wait_until/2
       test/electric/shape_cache_test.exs:1582: (test)

To view more test analytics, go to the Test Analytics Dashboard
_{📋 Got 3 mins? Take this short survey to help us improve Test Analytics.}

[robacourt]

Closed in favour of #4660 . We'll wait for beam-telemetry/telemetry_metrics_prometheus_core#77 to be merged before using it.