Add diagnostics_channel TracingChannel support

[jeremyhayes]

This PR introduces tracing channels support to pg-pool and pg querying methods as discussed in #3619

I haven't yet run a benchmark, but without active tracing or subscribers the overhead is non-existent so existing users shouldn't be affected. The overhead would come from the consumers of the diag channels published here which is done anyways via monkey patching.

I added the following channels:

Channel	Type	Package	Description
pg:query	TracingChannel	pg	Query lifecycle (start, end, error with async context)
pg:connection	TracingChannel	pg	Client connection lifecycle
pg:pool:connect	TracingChannel	pg-pool	Pool connection acquisition lifecycle
pg:pool:release	Channel	pg-pool	Client released back to pool
pg:pool:remove	Channel	pg-pool	Client removed from pool

I understand there may be hesitation given that AI tooling was used in the implementation. To be clear, this isn't a drive-by contribution. I designed the channel layout, wrote the proposal document, and iterated through multiple rounds to handle real edge cases (Node 18 compat, preserving custom Promise types, hasSubscribers guards). The AI assisted with the code, but the approach and decisions are mine.

Usage

Instrumentations will only need to subscribe to tracing channels to create traces, logs or metrics:

import { tracingChannel } from 'diagnostics_channel'

tracingChannel('pg:query').subscribe({
  start({ query, client }) {
    // start span
    // query: { text, name, rowMode }
    // client: { database, host, port, user, processID, ssl }
  },
  asyncEnd({ query, result }) {
    // end span
    // result: { rowCount, command }
  },
  error({ query, error }) {
    // record error on span
  },
})

This is part of a broader initiative to bring TracingChannel support to the most widely used Node.js database and cache libraries. The same pattern has already been merged and shipped in:

• mysql2
• node-redis
• ioredis

I have been directly involved in those prior implementations and I'm happy to own this through to release, address any feedback, and provide whatever support is needed. Would love to hear your thoughts.

Supersedes #3624.

[Copilot]

Pull request overview

This PR adds diagnostics_channel instrumentation for key pg and pg-pool lifecycles (query, connection, pool connect/release/remove), enabling consumers to build tracing/telemetry integrations without monkey-patching.

Changes:

• Introduces diagnostics helper modules for pg and pg-pool to safely initialize channels and gate tracing/publishing.
• Instruments pg Client#connect() and Client#query() using TracingChannel#traceCallback.
• Adds unit tests validating published events and context payloads for pg and pg-pool.

Show a summary per file

File	Description
packages/pg/test/unit/client/diagnostics-tests.js	Adds unit tests for pg:query and pg:connection tracing channels.
packages/pg/lib/diagnostics.js	Adds pg diagnostics channel initialization and shouldTrace gating helper.
packages/pg/lib/client.js	Wraps connect/query callbacks with tracing when subscribers exist.
packages/pg-pool/test/diagnostics.js	Adds unit tests for pool connect tracing and release/remove publish channels.
packages/pg-pool/index.js	Publishes pool lifecycle events via diagnostics channels.
packages/pg-pool/diagnostics.js	Adds pg-pool diagnostics channel initialization and shouldTrace gating helper.

Review details

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• Files reviewed: 6/6 changed files
• Comments generated: 4
• Review effort level: Low