[pkg/stanza] log when files are rotated/moved/truncated

[ChrsMark]

Description:

This PR adds the logging part from #31256.
With this addition every time that is identified that file is rotated
either by move/create or copy/truncate, proper logging takes place.

Link to tracking Issue: #31256

Testing:
Updated existing unit tests

How to test this manually

Using the following config:

receivers:
  filelog:
    start_at: beginning
    poll_interval: 5s
    include:
    - /var/log/busybox/monitoring/stable*.log

exporters:
  debug:
    verbosity: detailed

service:
  telemetry:
    logs:
      level: info
  pipelines:
    logs:
      receivers: [filelog]
      exporters: [debug]
      processors: []

Testing truncate (out of pattern)

echo "$(date '+%FT%H:%M:%S.%NZ') some line1" >> /var/log/busybox/monitoring/stable_trunc.log && 
echo "$(date '+%FT%H:%M:%S.%NZ') some line2" >> /var/log/busybox/monitoring/stable_trunc.log &&  
echo "$(date '+%FT%H:%M:%S.%NZ') some line3" >> /var/log/busybox/monitoring/stable_trunc.log && 
sleep 6 && 
cp /var/log/busybox/monitoring/stable_trunc.log /var/log/busybox/monitoring/stable_trunc.log.1 && 
: > /var/log/busybox/monitoring/stable_trunc.log && 
echo "$(date '+%FT%H:%M:%S.%NZ') some line new0" >> /var/log/busybox/monitoring/stable_trunc.log

Testing truncate (in pattern)

echo "$(date '+%FT%H:%M:%S.%NZ') some line1" >> /var/log/busybox/monitoring/stable_trunc.log &&
echo "$(date '+%FT%H:%M:%S.%NZ') some line2" >> /var/log/busybox/monitoring/stable_trunc.log && 
echo "$(date '+%FT%H:%M:%S.%NZ') some line3" >> /var/log/busybox/monitoring/stable_trunc.log &&
sleep 6 &&
cp /var/log/busybox/monitoring/stable_trunc.log /var/log/busybox/monitoring/stable_trunc_1.log &&
: > /var/log/busybox/monitoring/stable_trunc.log &&
echo "$(date '+%FT%H:%M:%S.%NZ') some line new1" >> /var/log/busybox/monitoring/stable_trunc.log

Testing move/create (out of pattern)

echo "$(date '+%FT%H:%M:%S.%NZ') some line1" >> /var/log/busybox/monitoring/stable_trunc.log &&
echo "$(date '+%FT%H:%M:%S.%NZ') some line2" >> /var/log/busybox/monitoring/stable_trunc.log && 
echo "$(date '+%FT%H:%M:%S.%NZ') some line3" >> /var/log/busybox/monitoring/stable_trunc.log &&
sleep 6 &&
mv /var/log/busybox/monitoring/stable_trunc.log /var/log/busybox/monitoring/stable_trunc.log.1 &&
echo "$(date '+%FT%H:%M:%S.%NZ') some line new0" >> /var/log/busybox/monitoring/stable_trunc.log

Testing move/create (in pattern)

echo "$(date '+%FT%H:%M:%S.%NZ') some line1" >> /var/log/busybox/monitoring/stable_trunc.log &&
echo "$(date '+%FT%H:%M:%S.%NZ') some line2" >> /var/log/busybox/monitoring/stable_trunc.log && 
echo "$(date '+%FT%H:%M:%S.%NZ') some line3" >> /var/log/busybox/monitoring/stable_trunc.log &&
sleep 6 &&
mv /var/log/busybox/monitoring/stable_trunc.log /var/log/busybox/monitoring/stable_trunc_1.log &&
echo "$(date '+%FT%H:%M:%S.%NZ') some line new0" >> /var/log/busybox/monitoring/stable_trunc.log

Documentation:
Add some extra notes in the design.md

[djaglowski]

Should we add actual test cases to make sure we don't regress? We could use zaptest/observer to validated the expected logs.

[djaglowski]

I suppose it doesn't technically matter but do you think it would be slightly more readable to use oldReader here, simply because we are communicating to the reader about the old file that is no longer here?

[djaglowski]

Good explainations. Can I suggest we reverse the order so that we start with the simplest case (move/create within pattern) and progress to the most complex (copy/truncate out of pattern)?

[djaglowski]

Here as well, I think move/create is simpler to understand since copy/truncate comes with a possible data loss scenario.