Add combine action

[james-bebbington]

Description:

Add a "combine" action to the Metrics Transform processor:

• Validates that all metrics to be combined have the same type, unit, and labelset
• Re-uses the existing data point aggregation functions to merge data points from different metrics
• Automatically appends labels extracted from the regexp filter to data points

See README & added test cases for more details.

Resolves #1088

[codecov]

Codecov Report

Merging #1506 (7f4a58a) into master (aa83481) will increase coverage by 0.00%.
The diff coverage is 98.88%.

@@           Coverage Diff           @@
##           master    #1506   +/-   ##
=======================================
  Coverage   88.71%   88.71%           
=======================================
  Files         346      347    +1     
  Lines       16934    16997   +63     
=======================================
+ Hits        15023    15079   +56     
- Misses       1443     1450    +7     
  Partials      468      468           

Flag	Coverage Δ
integration	70.80% <ø> (-0.08%)	⬇️
unit	87.37% <98.88%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...stransformprocessor/metrics_transform_processor.go	99.14% <98.00%> (-0.86%)	⬇️
processor/metricstransformprocessor/config.go	100.00% <100.00%> (ø)
...metricstransformprocessor/datapoint_aggregation.go	100.00% <100.00%> (ø)
processor/metricstransformprocessor/factory.go	98.79% <100.00%> (+0.02%)	⬆️
...sformprocessor/operation_aggregate_label_values.go	100.00% <100.00%> (ø)
...cstransformprocessor/operation_aggregate_labels.go	100.00% <100.00%> (ø)
receiver/prometheusexecreceiver/receiver.go	85.83% <0.00%> (-2.50%)	⬇️
...eiver/awsxrayreceiver/internal/udppoller/poller.go	97.67% <0.00%> (-2.33%)	⬇️
receiver/carbonreceiver/transport/tcp_server.go	65.34% <0.00%> (-1.00%)	⬇️

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[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[nilebox]

Do you validate that transform.AggregationType is set correctly? Similar to Action being one of 3 possible "enum" values.

[nilebox]

Also, should the function be named combineTimeseries, or action "merge" for consistency?

[james-bebbington]

Do you validate that transform.AggregationType is set correctly? Similar to Action being one of 3 possible "enum" values.

The way config is validated in this component is a little strange at the moment and there is validation missing for quite a few important cases. For now, I've added the obvious missing validation: on AggregationType & OperationAction.

This component probably needs a future PR to separate config into different types for different actions.

Also, should the function be named combineTimeseries, or action "merge" for consistency?

I think this is okay. One aspect of the "combine" action is merging timeseries. Merging timeseries is also relevant for the aggregating labels & aggregating label values actions.