Document that metric sinks aggregate metrics within a batch

[bruceg]

Most of the metric sinks aggregate metrics within a batch. That is, incremental metrics are added together and absolute metrics are replaced by the later value. As noted by https://github.com/timberio/vector/pull/7846#pullrequestreview-686985284 this is not adequately described in the documentation for those sinks. This applies to the following sinks:

• aws_cloudwatch_metrics
• datadog_metrics
• stackdriver_metrics
• influxdb_metrics
• prometheus_exporter
• prometheus_remote_write
• sematext_metrics

[moschroe]

I was tripped up by this when naively assuming I could import a backlog of metrics from CSV files to influxdb.

My workaround is specifying a batch.max_events: 1, which is terrible for performance, of course.

I my opinion, this aggregation should not occur in the first place, but that is a different discussion, I guess.

[StephenWakely]

Note stackdriver_metrics no longer aggregates within a batch.

[curtiseng]

How long before influxdb_metrics can support turning off auto-aggregation?