pgwire: add a metric for number of pipelined requests

[rafiss]

fixes #123912
Release note (ops change): Added the sql.pgwire.pipeline.count metric, which is a gauge that measures how many wire protocol commands have been received by the server, but have not yet begun processing. This metric will only grow if clients are using the "pipeline mode" of the Postgres wire protocol.

[cockroach-teamcity]

This change is 

[fqazi]

Just one minor question

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rafiss)

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pkg/sql/pgwire/conn_test.go line 300 at r1 (raw file):

	require.NoError(t, err)

	testutils.SucceedsSoon(t, func() error {

Is this because we need to wait for the queries above to finish in the pipeline? Could we do GetResults?

[rafiss]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @fqazi)

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pkg/sql/pgwire/conn_test.go line 300 at r1 (raw file):

Previously, fqazi (Faizan Qazi) wrote…

Is this because we need to wait for the queries above to finish in the pipeline? Could we do GetResults?

this is actually needed since we have to wait for the queries to reach the server and get into the pipeline.

GetResults wouldn't work here since the fake server in this test never actually sends results back to the client. and the reason we need a fake server is so that we can control exactly when the stmtBuf is read from.

i'll add some comments

[rafiss]

bors r+

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @fqazi)

[rafiss]

tftr!

bors r+

[craig]

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[craig]

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