[metricstarttimeprocessor] Add start_time_metric strategy

[dashpole]

Component(s)

processor/metricstarttime

Is your feature request related to a problem? Please describe.

The Prometheus receiver can use the "process_start_time" metric to set the start time of metrics when the start time is not known. In order to provide a migration path for users off of that, we should implement a start_time_metric strategy in the processor.

Describe the solution you'd like

When this strategy is selected, it should default to looking for the process_start_time metric, similar to the Prometheus receiver. It should support the same configuration options the prometheus receiver supports. This includes being able to fall back to the collector's start time, or selecting a different start time metric.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners for processor/metricstarttime: @dashpole @ridwanmsharif. See Adding Labels via Comments if you do not have permissions to add labels yourself. For example, comment '/label priority:p2 -needs-triaged' to set the priority and remove the needs-triaged label.

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

Pinging code owners:

• processor/metricstarttimeprocessor: @dashpole @ridwanmsharif

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[dashpole]

I looked into whether it is possible to replace this functionality using the transform processor, but it doesn't seem like it. We would need to be able to define an OTTL function which:

• Records the value of a metric with a particular name in a "local variable".
• Sets the start time of all metrics without a start time to that local variable.

I don't believe OTTL has the concept of local variables, so I'm going to implement this in the metricstarttimeprocessor