Clear methods when Gunicorn worker exits

[adriangonz]

What this PR does / why we need it:

Clear the internal Python server metrics when a Gunicorn worker exists. Otherwise, the internal dictionary keeps growing, causing a high memory usage in the server itself and other downstream issues in Prometheus.

Which issue(s) this PR fixes:

Fixes #2873

Special notes for your reviewer:

Only the entries relevant to a worker process will be cleared.

Does this PR introduce a user-facing change?:

NONE

[adriangonz]

/cc @axsaucedo @RafalSkolasinski

[RafalSkolasinski]

/test notebooks
/test integration

[RafalSkolasinski]

Nice one!

Just have a few minor questions, see inlines.

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RafalSkolasinski

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[RafalSkolasinski]

Also the lint is failing

[anggao]

Thanks! This looks great I will test it next week!

[RafalSkolasinski]

For some reason tests were again triggered twice. I see one set of integration and notebooks to passed:

[seldondev]

@adriangonz: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
notebooks	af97eda	link	/test notebooks
integration	af97eda	link	/test integration

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