Validation and narrower try/except in BenchmarkMetric

[esantorella]

Summary: Context:

BenchmarkMetric.fetch_data originally had a broad try/except block just because this the usual way to define metrics. The try/except is actually not needed at all; with benchmarking, there are no potentially-flaky RPCs and we completely understand what should happen when fetching data. BenchmarkMapMetric has no try/except. The following cases are possible when fetching data:

• If the trial has not been run, it will have no metadata, and there should be a MetricFetchE (error).
• If a trial has been run, it should have properly formatted metadata, and fetch_data should succeed.
• If there is a problem with the metadata, this really shouldn't happen, and fetch_data should form with an informative error.

This PR:

• Unifieds the logic between BenchmarkMetric and BenchmarkMapMetric so that both return a MetricFetchE when there is no metadata and do not have a try/except
• Pulls out a mixin for code-sharing

Differential Revision: D66301697

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66301697

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.43%. Comparing base (5d1e441) to head (6d512ca).

Files with missing lines	Patch %	Lines
ax/benchmark/benchmark_metric.py	94.11%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3100 +/- ## ========================================== + Coverage 95.41% 95.43% +0.01% ========================================== Files 495 495 Lines 49941 49956 +15 ========================================== + Hits 47653 47674 +21 + Misses 2288 2282 -6 ```

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D66301697

[facebook-github-bot]

This pull request has been merged in facebook/Ax@095d7ed2b59716ec10e387b0705a7bd90547d869.