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