[8.x](backport #41667) filebeat/inputs/filestream: add metric for messages truncated

[mergify]

Proposed commit message

While investigating an SDH, I noticed that although we add the truncated label to log fields, there is no feedback on the amount of messages that are truncated. It often happens that almost all messages are truncated because of a misconfigured input.

This PR adds a input metric that counts the total number of truncated messages.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Create a log file with multiline logs:

cat <<EOF > /tmp/in.log
[beat-logstash-some-name-832-2015.11.28] IndexNotFoundException[no such index]
    at org.elasticsearch.cluster.metadata.IndexNameExpressionResolver\$WildcardExpressionResolver.resolve(IndexNameExpressionResolver.java:566)
    at org.elasticsearch.cluster.metadata.IndexNameExpressionResolver.concreteIndices(IndexNameExpressionResolver.java:133)
    at org.elasticsearch.cluster.metadata.IndexNameExpressionResolver.concreteIndices(IndexNameExpressionResolver.java:77)
    at org.elasticsearch.action.admin.indices.delete.TransportDeleteIndexAction.checkBlock(TransportDeleteIndexAction.java:75)
EOF

Configure filebeat to parse the multiline logs with max_lines:

filebeat.inputs:
- type: filestream
  id: foo-bar
  enabled: true
  paths:
    - "/tmp/in.log"
  parsers:
    - multiline:
        type: pattern
        pattern: '^\['
        negate: true
        match: after
        max_lines: 3

output.discard:
  enabled: true
http.enabled: true

Check the metrics:

$ curl -XGET 'localhost:5066/inputs/' | jq
[
  {
    "bytes_processed_total": 596,
    "events_processed_total": 1,
    "files_active": 1,
    "files_closed_total": 0,
    "files_opened_total": 1,
    "id": "foo-bar",
    "input": "filestream",
    "messages_read_total": 1,
    "messages_truncated_total": 1,
    "processing_errors_total": 0,
    "processing_time": {
      "histogram": {
        "count": 1,
        "max": 5000516503,
        "mean": 5000516503,
        "median": 5000516503,
        "min": 5000516503,
        "p75": 5000516503,
        "p95": 5000516503,
        "p99": 5000516503,
        "p999": 5000516503,
        "stddev": 0
      }
    }
  }
]

Related issues

---

This is an automatic backport of pull request #41667 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 7806f1a has failed:

On branch mergify/bp/8.x/pr-41667
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 7806f1a2c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   filebeat/docs/inputs/input-filestream.asciidoc
	modified:   filebeat/input/filestream/environment_test.go
	modified:   filebeat/input/filestream/input.go
	modified:   filebeat/input/filestream/internal/input-logfile/metrics.go
	modified:   libbeat/reader/multiline/message_buffer.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   filebeat/input/filestream/metrics_integration_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)