[DOCS]: need to add documentation for extract line numbers in metrics

[shahzebsiddiqui]

Documentation Link

https://buildtest.readthedocs.io/en/devel/writing_buildspecs/metrics.html

Issue Description

@Xiangs18 can you please add docs for this PR https://github.com/buildtesters/buildtest/issues/1725

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[Xiangs18]

Should I start a new section in Docs? I assume file_regex would required some documentation as well?

[shahzebsiddiqui]

Docs

Yes just add this as a new subsection after https://buildtest.readthedocs.io/en/devel/writing_buildspecs/metrics.html#metrics-with-regex-type-via-re. You can have one section for extracting line number and use both file_regex and regex. If you look at the content of this page, we already cover file_regex in the example so its not required to just have a section to explain what file_regex is. This is already covered in the page. I just want you to document how one can extract a particular line using the linenum property.