Force users to always specify a tag when creating a MultiProcessShared object (breaking)

[damccorm]

Right now, we don't require a value for the mps tag. This means that the default behavior (if a user doesn't know what they're doing) is to not actually share the object across processes; instead each process will create an object for that process.

One downside of this is that this change does make it harder to have a single process that proxies all requests to a separate process (which is the current behavior if you specify no tag and alwaysProxy=true). If a user really wants that use case, they can do something like abc=MultiProcessShared(<constructor>, tag=uuid.uuid().hex, alwaysProxy=true) to get the same behavior. I think that will be a much rarer use case though.

This change is intentionally breaking because if anyone is currently using this feature they're probably actually intending to share the object. Plus this whole thing was sorta busted in the mainline case anyways (see #26172)

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[damccorm]

Looks like tests are going to fail until we have https://github.com/apache/beam/actions/runs/4640195969/jobs/8211875305?pr=26178 (which is good evidence that we need it 😃)

[codecov]

Codecov Report

Merging #26178 (81eb884) into master (f549fd3) will decrease coverage by 9.29%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master   #26178      +/-   ##
==========================================
- Coverage   81.12%   71.83%   -9.29%     
==========================================
  Files         469      752     +283     
  Lines       67185   101419   +34234     
==========================================
+ Hits        54501    72851   +18350     
- Misses      12684    27086   +14402     
- Partials        0     1482    +1482     

Flag	Coverage Δ
python	81.08% <94.11%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/runtime/exec/status.go	0.00% <ø> (ø)
...kg/beam/core/runtime/graphx/schema/logicaltypes.go	69.23% <ø> (ø)
...s/go/pkg/beam/core/runtime/graphx/schema/schema.go	65.94% <55.55%> (ø)
sdks/go/pkg/beam/core/runtime/exec/plan.go	70.00% <56.25%> (ø)
...s/python/apache_beam/utils/multi_process_shared.py	96.92% <94.11%> (-0.52%)	⬇️

... and 293 files with indirect coverage changes

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[damccorm]

R: @robertwb

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[damccorm]

Note that merging in master with #26172 fixed the previously failing test