Consider adding exemplars to metrics

This issue has been migrated from [#15927](https://github.com/matrix-org/synapse/issues/15927).

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c.f. https://grafana.com/docs/grafana/latest/fundamentals/exemplars/

These allow you to send up "examples" of various metrics, that can be rendered on graphs (with links to jaeger).

This requires opt-in in both prometheus (via a feature flag), and grafana in the data source and graph settings.

I'm not sure if we want to a) do this by default, or b) make it optional. I don't know how Prometheus handles exemplars if they're not enabled, and how older versions handle it (certainly prometheus fails to parse the metrics if you don't use the right openmetrics content type for the response)

I've got a WIP branch at https://github.com/matrix-org/synapse/compare/erikj/exemplars, with a screen shot below of it working.