Add pgx pool acquire tracer

[ngavinsir]

resolves #1747

[jackc]

Looks pretty good, but I have a few suggestions.

I think that TraceAcquireStart and TraceAcquireEnd should receive the *Pool as an argument. This would match the precedent set by QueryTracer of passing the item that is being traced. This would also mean that TraceAcquireStartData would not need to include ConnConfig. TraceAcquireStartData would still exist for possible future changes, but it would be empty.

It's a little confusing at first whether TraceAcquireEnd is called when the connection is acquired or when the acquisition is complete (i.e. when it is released). I think the docs should make that clearer. TraceAcquireEndData should probably also include the *pgx.Conn.

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This might be a separate feature, but it might also make sense to have a ReleaseTracer interface to allow tracing the entire time the the connection is acquired. In addition to the normal metrics usage of tracing, this would allow building diagnostics that would make it easier to track down connection leaks.

[ngavinsir]

@jackc updated, but not sure how to pass the current context to the release method, so it can have the same parent trace as the acquire one. Is it ok to save it in the conn struct?

[jackc]

@ngavinsir I guess the release tracer is a bit trickier than I thought.

It is, perhaps, a little weird to use the context from Acquire for the Release. But I suppose context is where things like opentelemetry are stored so it is necessary. 🤔 At least it should be documented that it is the Acquire Context and/or made part of TraceRelease(Start|End)Data. Storing it in the pgxpool.Conn is fine. It's private so we can refactor it if necessary.

I also wonder if release tracing needs Start and End. Release should be consistently extremely fast and non-blocking. Perhaps it only needs one hook?

[jackc]

🤔 And on even further reflection, maybe the release tracer doesn't need context at all. As of 6f0deff there is a custom data map available on connections. Any release tracer that needed it could simply store it there.

[ngavinsir]

@jackc I see, so for example someone can save the passed context on TraceAcquireEnd in the TraceAcquireEndData.Conn.PgConn().CustomData() and access it again inside trace release from TraceReleaseData.Conn.PgConn().CustomData()

[jackc]

Thanks!