Docs: Filtering Metrics in Surfacers

[robpickerill]

Staging this PR to align filtering docs as a follow on for #597. I'd spotted the cloudwatch docs needed a quick update so I took the opportunity to also write a bit of documentation for filtering metrics.

I'd also spotted I'd missed the surfacers overview -> cloudwatch reference so added that here.

There is a minor code change in the protobuf config and tests for cloudwatch so I think this PR needs splitting again? Let me know what's best.

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[manugarg]

Thanks for this. One minor suggestion.

[manugarg]

@robpickerill Can you remove protobuf and Go change from this PR. Submitting them through:
#600

[manugarg]

@robpickerill Any chance you can squash your commits into one? I can try doing it but it will be another pull request / direct commit (with you still as author) again.

[robpickerill]

@robpickerill Any chance you can squash your commits into one? I can try doing it but it will be another pull request / direct commit (with you still as author) again.

I squashed and reworded the commit message for the cleaner commit history - apologies for the messy history, but now fixed :)