Add transfer_outgoing_bytes_total metric

[gjoseph92]

Closes #7123

cc @hendrikmakait @crusaderky @ntabris

• Tests added / passed
• Passes pre-commit run --all-files

[ntabris]

FYI _total will be added to counter metrics so it's not necessary to have in name but also doesn't hurt: https://github.com/prometheus/client_python#counter

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       18 files  ±0         18 suites  ±0   7h 48m 2s ⏱️ - 34m 11s
  3 254 tests ±0    3 166 ✔️ ±0       86 💤 +1  2 ❌  - 1 
29 295 runs  ±0  28 067 ✔️ +3  1 226 💤  - 2  2 ❌  - 1 

For more details on these failures, see this check.

Results for commit 7249762. ± Comparison against base commit 653b006.

♻️ This comment has been updated with latest results.

[hendrikmakait]

Thanks @gjoseph92, this generally looks good to me. I'd slightly change the wording to clarify what we measure here.

Additionally, I wonder if we should extend this metric with a state label with three states like attempted, succeeded and failed. This would allow us to resolve the issue mentioned in #6936 (comment) and would allow us to drop the corresponding gauge, i.e., requested - (succeeded + failed) = in-progress. I'd leave this for future work though since it can be generalized to all the transfer metrics. (I assume that we are fine with totally breaking changes for new metrics, see #7385.)

@ntabris' nit around _total matches what I propose in #7374, but it doesn't hurt for now.