Add MetricsContainer support to the Flink sources.

[jto]

This is an updated version of the abandonned #25753.
I simply rebased master in the original PR and reviewed the code.

fixes #25741

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[jto]

The failing test is running fine on my machine. I think the test is just flaky. It would probably succeed if someone just re-triggers it.

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[jto]

assign set of reviewers

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[kennknowles]

Seems good to me generally. I made coding comments but I don't want to stand in the way of making things better, since it is it all under-the-hood stuff anyhow.

[jto]

@kennknowles I made the fix you requested. The test fails when publishing the results with the following error:

Error: Error processing result file: CData section too big found, line 92832, column 19 (TEST-org.apache.beam.sdk.transforms.ViewTest.xml, line 92832)

It seems unrelated to this change. Would you happen to know if I can do anything to fix this problem ?

[tvalentyn]

clicked to rerun the tests, maybe a flake?

[jto]

Thanks @tvalentyn. It failed with the same error so I guess it's deterministic...

[github-actions]

Reminder, please take a look at this pr: @johnjcasey

[tvalentyn]

waiting on author

[tvalentyn]

Actually, looking at the test execution, it passed:

BUILD SUCCESSFUL in 31s
103 actionable tasks: 64 executed, 39 from cache

Publishing build scan...
https://ge.apache.org/s/a7mu5k567st5o

and fails later. I am fine with filing a separate issue about test reporting, and merge this.

[tvalentyn]

@jto do you prefer to merge it or you'd like to work more on 4296688 ? Perhaps we should move out that commit into a separate PR to make sure we don't make the situation worse in this PR.

[jto]

@tvalentyn thanks!
I don't really know how to fix the build failure. It indeed fails when publishing the build results while all tests are green. I don't know why this PR in particular breaks the build. Seems to be something about the size of an XML file so if you wish to merge this and address the issue in another PR, please feel free to do it :) I'm sure someone more familiar with the build itself will know how to fix this.

[github-actions]

Reminder, please take a look at this pr: @johnjcasey