Add DownsampleMetrics

[kkrik-es]

A single instance gets created by the Downsample plugin and injected to related actions.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[kkrik-es]

Another option is to return (inject) the DownsampleMetrics object in Downsample::createComponents, so that it can be retrieved in actions. This lifts the need for a singleton but will still require updating a number of intermediate classes until we get to the code that records a metric value.

[martijnvg]

LGTM - I thought we have some sort of a unit test infrastructure that we can use for metrics. @salvatore-campagna do you remember?

[kkrik-es]

LGTM - I thought we have some sort of a unit test infrastructure that we can use for metrics. @salvatore-campagna do you remember?

https://github.com/elastic/elasticsearch/blob/main/modules/apm/METERING.md#testing says we use TestTelemetryPlugin, not sure if there's a better way..

[martijnvg]

says we use TestTelemetryPlugin, not sure if there's a better way..

Ok, I see. And this is already tested in the changes to the unit test in this pr.