Add SQL server log file size metrics

[nwneisen]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

Relates to #4382

[danielnelson]

@m82labs @denzilribeiro Can you take a look?

[denzilribeiro]

Went through the pull request effectively we are adding a query to get 2 things but not sure we are achieving much.
a. sum space for all files on disk ( not used) - This should already be there in performance countner format.
b. max space for log

This PR doesn't achieve what you want theoretically - log can be 99% empty so just looking at size on disk isn't helpful - Even if size on disk = max size but 99% is free why would it matter?

Besides you have Data File and Log file size form the performance counters already per database in the sqlserver_performance measurement.I can add size on disk to existing collector already, aka where we get the IO numbers from virtual filestats if the perfmon number isn't enough. Ideally would be great to get used space from all files but that is much tricker given it requires DB context to do something FILEPROPERTY(file_name,'UsedSpace') or have to write more complex query to loop through databases - not sure worth effort or if worth it has to be at a different frequency totally not every 10 seconds by default.