backuppb: fix incorrect ExportRequest throughput aggregation

[adityamaru]

ExportStats are emitted on the return of every ExportRequest. A tracing aggregator on each backup processor listens for these events and maintains a running aggregate. This aggregate is then used to generate interesting metrics, one of which is the throughput in MB/sec at which this processor is processing ExportRequests.

Previously, we were naively using the aggregatedBytes / aggregatedDuration to compute this thorughput. This is incorrect because we process several ExportRequests concurrently per processor. To account for this we now maintain the earliest start time and the latest end time across the requests we have aggregated. When computing throughput we use the difference between these two timestamps as our denominator.

Fixes: #89579

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[adityamaru]

TFTR!

bors r=rhu713

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 602e27c to blathers/backport-release-22.2-96968: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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