Use consistent identity not per-request UUID for worker IDs

[agrski]

What this PR does / why we need it:
All the metrics from the alibi-detect-server are unable to be used as-is for monitoring and alerting across the span of multiple events.

The use of a UUID means every request is treated as a new timeseries by Prometheus because the labels are different every time.
Furthermore, it means metrics, whether gauges or counters, can never be reset, which is painful for alerting.

The default worker ID function--the process ID for the worker--will be consistent across requests, which mitigates both of the aforementioned issues.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fixed alibi-detect-server metrics to allow effective alerting by providing a consistent set of labels across every event.

[seldondev]

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[axsaucedo]

Nice one
/test integration

[seldondev]

@agrski: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
integration	e999345	link	/test integration

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[agrski]

/test integration

[agrski]

From testing locally, I can't see anything wrong so not sure why the integration tests complained before:

$ make test
mypy --ignore-missing-imports adserver
Success: no issues found in 29 source files
pytest -W ignore adserver
==================================================================== test session starts ====================================================================
platform linux -- Python 3.8.12, pytest-6.2.5, py-1.11.0, pluggy-1.0.0
rootdir: <...>/seldon-core/components/alibi-detect-server
plugins: tornasync-0.6.0.post2, requests-mock-1.9.3
collected 14 items                                                                                                                                          

adserver/tests/test_ad_model.py ..                                                                                                                    [ 14%]
adserver/tests/test_cd_model.py ...                                                                                                                   [ 35%]
adserver/tests/test_cm_model.py ...                                                                                                                   [ 57%]
adserver/tests/test_od_model.py ..                                                                                                                    [ 71%]
adserver/tests/test_server.py ....                                                                                                                    [100%]

============================================================== 14 passed in 107.74s (0:01:47) ===============================================================