loki.source.docker: update to use shared scheduler

[kalleep]

PR Description

loki.source.docker suffered from the same issue that loki.source.file did where scheduling could take some time for new targets.

In this pr I move Scheduler to source package so I can be reused by both components. With this I also moved "target" into docker package and rename to tailer, this tailer now implements Source interface.

I also fixed a issue where stopping component could deadlock if nothing was reading from handler chan.

Which issue(s) this PR fixes

Related to: #4729

Notes to the Reviewer

• Moved target and metrics from internal package to tailer.go and metrics.go and they are no longer exported.

• Created a shared structure Fanout, this implements the common pattern to abort send operation if context is canceled.

• Create shared function Consume to run the consume loop that will abort if context is canceled.

• Create shared function Drain that can be used when component is stopped.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated