Proper logging level extraction from exporters log messages

User story:

Currently for extracting log level from log message we are searching for the firs entrance of `level=` pattern with regexp. Which should work in most cases, however this approach can't properly handle this:

ts=2022-06-14T21:50:00.817Z caller=node_exporter.go:184  msg="naming collision level=error " level=info 

Instead of `info` our implementation will extract `error` level. We should investigate how proper logs parsing will affect CPU usage and if it's acceptable switch to it.

Here is some libraries for logfmt format parcing:
https://github.com/go-logfmt/logfmt
https://github.com/kr/logfmt

UI/UX:

Acceptance criteria:

Out of scope:

Suggested implementation:

How to test:

Details:

https://github.com/percona/pmm/pull/936