chore: test custom metrics with github-action-benchmark.

[RodrigoVillar]

One way to potentially have multiple metrics show up in our dashboard would be to switch from printing benchmark results via b.ReportMetric() to utilizing github-action-benchmark's custom metric format. If this works, this could save us from having to fork github-action-benchmark and fixing the bug that caused the dashboard corruption.

[RodrigoVillar]

Attached is a sample dashboard that uses custom benchmarks: https://rodrigovillar.github.io/gab-go-benchmark/dev/bench/

With a proof-of-concept established, the next step is to migrate from using go bench to using custom benchmarks in the reexecution test.

[RodrigoVillar]

Closing this issue as we've determined that we can use custom benchmarks instead of go bench. #4508 will track the progress of switching to using custom benchmarks.