Add option to set Counter to be monotonic

[locmai]

Description:
Detailed bug: #4153

This will provide the option to set counter metric to become monotonic and therefore become cumulative (AggregationTemporalityCumulative) instead of AggregationTemporalityDelta - which will be dropped by prometheusexporter.

Link to tracking Issue: #4153

Testing:

Build and run the otel-collector with the following config.yaml configuration:

receivers:
  statsd:
    endpoint: "0.0.0.0:8127"
    aggregation_interval: 5s
    enable_metric_type: true
    is_monotonic_counter: true
    timer_histogram_mapping:
      - statsd_type: "histogram"
        observer_type: "summary"
      - statsd_type: "timing"
        observer_type: "summary"
processors:
  batch:
exporters:
  prometheus:
    endpoint: "0.0.0.0:9090"
    metric_expiration: 180m
  file:
    path: ./metrics.json
service:
  pipelines:
    metrics:
      receivers: [statsd]
      processors: [batch]
      exporters: [prometheus,file]

Send the test command:

echo "locmai.test:30|c" | nc -w 1 -v -u 0.0.0.0 8127

The metric will be exported on prometheus exporter and file exporter as well:

metrics.json:

{"resourceMetrics":[{"resource":{},"instrumentationLibraryMetrics":[{"instrumentationLibrary":{},"metrics":[{"name":"locmai.test","intSum":{"dataPoints":[{"labels":[{"key":"metric_type","value":"counter"}],"timeUnixNano":"1626103559426960000","value":"30"}],"aggregationTemporality":"AGGREGATION_TEMPORALITY_CUMULATIVE","isMonotonic":true}}]}]}]}

prometheus:

# HELP locmai_test 
# TYPE locmai_test counter
locmai_test{metric_type="counter"} 30

Documentation:
I updated the option in README.md of the statsdreceiver

[bogdandrutu]

Is this true all the time? Do we need to have per counter config?

[locmai]

Do we need to have per counter config?

With the current change, it applies to all counters.

I think we do need for per counter config, we could add include/exclude with regex? Although it feels more like a processor job.

[jmacd]

Yes. In previous PRs on this receiver I've asked that we follow the Prometheus statsd exporter's configuration, e.g.,
https://github.com/prometheus/statsd_exporter#glob-matching

I think it is fine to add this to a TODO and continue in another PR. The current PR allows a default choice, and I would say that probably the default should be true (i.e., assume monotonic by default).

[locmai]

Yeah it would be great if this receiver were following the statsd exporter 👍🏼

May I have this one merged until that come? I set the default to false since seeing this #1789 so I don't want to cause any trouble for the DataDog exporter.