Add metrics metadata to prometheus remote write payload

## Feature Request
Send metric metadata to prometheus remote write

### Proposal:
Periodically send metric metadata to prometheus remote write endpoint.

### Current behavior:
The current serializer for prometheus remote write handles only the Timeseries data and does not send the metric metadata.
https://github.com/influxdata/telegraf/blob/master/plugins/serializers/prometheusremotewrite/prometheusremotewrite.go#L239

### Desired behavior:
A new option that specifies the interval of sending Metadata for Timeseries needs to be added to prometheus remote write config. If this option is configured, telegraf should add the metadata to prompb payload while sending the metrics to prometheus remote write endpoint.

### Use case:
In some of the monitoring tools (e.g Instana), metrics ingestion through prometheus remote write cannot be performed unless the metric metadata is transmitted along with the timeseries.

Start in plugins/serializers/prometheusremotewrite/prometheusremotewrite.go at the current serializer implementation around line 239. Trace how Timeseries data becomes the prompb payload, then determine how the proposed configurable metadata interval should control metadata transmission. Done means configured remote-write payloads include metric metadata at the requested interval without changing the existing timeseries behavior.

Written by the indexing model from the issue text.