Support set metrics

[Dav1dde]

There is currently no way to emit set metrics via metrics-rs.

For example, statsd and Sentry support sets natively.

[tobz]

This is something I'm not looking to support with metrics, as it can be more easily handled by applications themselves, and tracked directly as a gauge.

[Dav1dde]

How would the application track a set as a gauge? This does not work for anything distributed.

[tobz]

You're right: it would only be aggregated at the application-level, so perhaps by pod or by host. This is also how most metrics platforms that "support" StatsD (and thus sets) do it, though: They don't natively send the set data to the backend and do global aggregation, they just roll up the values locally and emit a gauge.

[Dav1dde]

That's unfortunate for all platforms that do support proper sets. But fair enough.