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

Adds diagnostics_channel TracingChannel/Channel instrumentation to node-postgres query/connect flows and pg-pool connect/release/remove lifecycles, plus unit tests validating emitted contexts/events.

Changes:

• Introduces diagnostics helper modules for pg and pg-pool that initialize tracing/publish channels when available.
• Wraps Client#connect and query enqueue/callback flow with TracingChannel.traceCallback to emit lifecycle events with context.
• Adds new unit tests for pg and pg-pool diagnostics channels (with runtime skipping for older/unstable Node versions).

Show a summary per file

File	Description
packages/pg/lib/client.js	Adds TracingChannel.traceCallback wrappers for connect and query callback lifecycles.
packages/pg/lib/diagnostics.js	Initializes pg:query and pg:connection tracing channels and exports shouldTrace().
packages/pg/test/unit/client/diagnostics-tests.js	Adds unit tests validating pg:query and pg:connection tracing events/contexts.
packages/pg-pool/index.js	Publishes pg:pool:* diagnostics events on connect/release/remove paths.
packages/pg-pool/diagnostics.js	Initializes pg:pool:connect tracing channel and pg:pool:release/remove channels; exports shouldTrace().
packages/pg-pool/test/diagnostics.js	Adds mocha tests for pool diagnostics channels (skipping tracing tests on older Node versions).

Review details

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• Files reviewed: 6/6 changed files
• Comments generated: 3
• Review effort level: Low