Adding documentation page on what ElastAlert metrics are being exposed via --prometheus_port configuration flag

[xavigpich]

Description

This PR adds a new documentation page that explains what types of metrics can be exported when using the --prometheus_port configuration flag

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Comments

• This is a documentation change, it does not require a unit test
• I've tested it locally by running make html && open build/html/recipes/exposing_rule_metrics.html

[ferozsalam]

@thisisxgp you're getting a build rule error:

Warning, treated as error:
/home/elastalert/docs/source/recipes/exposing_rule_metrics.rst:4:duplicate label writingrules, other instance in /home/elastalert/docs/source/recipes/adding_rules.rst

[xavigpich]

@thisisxgp you're getting a build rule error:

Warning, treated as error:
/home/elastalert/docs/source/recipes/exposing_rule_metrics.rst:4:duplicate label writingrules, other instance in /home/elastalert/docs/source/recipes/adding_rules.rst

Updated recipe page label to fix build error

[jertel]

@thisisxgp Thanks for submitting this new documentation. Will you please add a link to your new page from the running_elastalert.html page, specifically the --prometheus_port parameter?

[xavigpich]

@thisisxgp Thanks for submitting this new documentation. Will you please add a link to your new page from the running_elastalert.html page, specifically the --prometheus_port parameter?

Added link to new documentation page