Report Metrics from filestream input

[lprakashv]

Describe the enhancement:
Add some some metrics to report the internal health of filestream input.

Describe a specific use case for the enhancement or feature:
There are a lot of metrics being reported by log input but since that has been deprecated and migrating to filestream input is highly recommended. Some of the older filebeat.harvester.* and other metrics have become useless. We need some metrics for filestream input equivalent to:

• filebeat.input.log.files.truncated
• filebeat.harvester.running
• filebeat.harvester.skipped
• and other valuables metrics (started, closed, stopped etc.)

[botelastic]

This issue doesn't have a Team:<team> label.

[mikeh-elastic]

The metrics for /datastream are also absent when filestream is used compared to the log input.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!