Fix batch get metrics limit in reports service

[tristanvuong2021]

No description provided.

[wfa-reviewable]

This change is 

[SanjayVas]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @tristanvuong2021)

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src/main/proto/wfa/measurement/reporting/v2alpha/metrics_service.proto line 95 at r1 (raw file):

    }
  ];
  // A maximum of 1000 metrics can be retrieved in a batch.

I'm assuming that this limit is reasonable, i.e. all of the following are true:

• The RPC can complete in a reasonable amount of time
• A response with 1000 Metrics is unlikely to exceed the 4 MiB response size limit
• This won't cause undue pressure on machine resources