Add fetchMetrics API to BaseStatsReporter

[majetideepak]

Return the aggregated metrics in a serialized string format.

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[majetideepak]

@tanjialiang can you help review/land this change? Thanks!

[xiaoxmeng]

@majetideepak why we need this? thanks!

[majetideepak]

@xiaoxmeng We need this to simplify the prometheus metrics implementation in Presto.
See @tanjialiang's comment here prestodb/presto#22360 (comment).
Without this, we need to add a custom implementation something like.

std::string getSerializedMetrics() {
  if (auto reporter =
          std::dynamic_pointer_cast<prometheus::PrometheusStatsReporter>(
              folly::Singleton<velox::BaseStatsReporter>::try_get())) {
    return reporter->getSerializedMetrics();
  } else if (...) {
  }
  VELOX_UNREACHABLE("No metrics reporter is enabled");
}

[majetideepak]

@xiaoxmeng, @tanjialiang any thoughts on this API? Thanks!

[tanjialiang]

Thanks for the change. Some minor issues.

[tanjialiang]

/// Return the aggregated metrics in a serialized string format.

[tanjialiang]

I'm wondering if this API should be const. Depending on implementations, clean-like operations can happen inside per each fetch interval.

[majetideepak]

Makes sense to remove the const. I address the comments. Thanks!

[facebook-github-bot]

@tanjialiang has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@tanjialiang merged this pull request in 0fe2e62.

[conbench-facebook]

Conbench analyzed the 1 benchmark run on commit 0fe2e621.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details.