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 native diagnostics-channel tracing to pg and pg-pool.

Changes:

• Instruments query, connection, and pool acquisition lifecycles.
• Publishes pool release and removal events.
• Adds diagnostics tests and compatibility fallbacks.

Show a summary per file

File	Description
packages/pg/lib/diagnostics.js	Defines pg tracing channels.
packages/pg/lib/client.js	Instruments connections and queries.
packages/pg/test/unit/client/diagnostics-tests.js	Tests client diagnostics.
packages/pg-pool/diagnostics.js	Defines pool diagnostics channels.
packages/pg-pool/index.js	Instruments pool lifecycle events.
packages/pg-pool/test/diagnostics.js	Tests pool diagnostics.

Review details

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Comments suppressed due to low confidence (1)

packages/pg/lib/client.js:231

• The promise branch has the same raw-payload issue: _connect passes the Client as its second callback argument, and traceCallback publishes it as context.result. The tracing context should receive only the documented connection metadata while the internal resolver continues to resolve with this.

        connectionChannel.traceCallback((tracedCb) => this._connect(tracedCb), 0, context, null, callback)

• Files reviewed: 6/6 changed files
• Comments generated: 7
• Review effort level: Medium