Add MetricsContainer support to the Flink sources.

[becketqin]

This patch is a follow-up patch for the Flink runner migration from DataSet API to DataStream API. It adds support for MetricsContainer which exposes the metrics defined in Beam metrics via the Flink metric framework.

fixes #25741

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[becketqin]

@xinyuiscool Would you help take a look? Thanks!

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[tvalentyn]

@becketqin are you still interested in submitting this change? I'd recommend reaching out on @dev and ask folks who are familiar with FlinkRunner codebase to help take a look.

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[tvalentyn]

@becketqin are you still interested in submitting this change? I'd recommend reaching out on @dev and ask folks who are familiar with FlinkRunner codebase to help take a look.

ping on this. Thanks!

[jto]

Hey there!
Can someone take at look at this PR and #25742 ?
Looks like everything is in place for Beam use DataStream in place of the deprecated DataSet API.
Maybe @xinyuiscool since you reviewed and merged #25525 :)

[tvalentyn]

hey @jto, thanks for reaching out. unfortunately I haven't heard from the original author - if you or someone else can take over the PRs, it might be a path to get the change unblocked. In addition to an author, we will need a reviewer who is familiar with the codebase or is willing to dig in to take more ownership in the flink runner codebase. You could also be a reviewer, then, we will need a committer for the merge but I this point I think we are looking for review to complete. reaching out to dev@ may be an avenue to get help.

[jto]

Hey @tvalentyn ! Thanks for your reply. I think either one of my colleague or myself will take over those two PRs :)

[tvalentyn]

Thanks, @jto, let's continue in #28609