server: populate WireLength on stats.InPayload for unary RPCs

[ajwerner]

Fixes #2692 which was incompletely fixed by #2711.

Also adds a basic unit test which provides scaffolding for further testing of
the stats logic.

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[ajwerner]

@menghanl any chance I can bother you to review this PR to finish off fixing #2692?

[menghanl]

Thanks for the fix. The change looks good.

For the test, can you see if it can be done in stats_test.go instead?
Maybe a WireLength check here for in payload and here for out payload

[ajwerner]

For the test, can you see if it can be done in stats_test.go instead?
Maybe a WireLength check here for in payload and here for out payload

Done. Much simpler.

Any hope we could backport this and #2711 to 1.21?

[menghanl]

We can do a backport for this change.

Isn't #2711 already in 1.21?

[ajwerner]

We can do a backport for this change.
Awesome!

Isn't #2711 already in 1.21?

Yes, sorry, I've been getting my versions mixed up. I was looking at 1.18 but if we get this into 1.21 I'm happy. I'll make the PR.