Fix prometheus_scrape integration by adding prometheus_alert_rules

[amandahla]

Overview

If there is no prometheus_alert_rules the charm fails with "Invalid Prometheus alert rules folder at /var/lib/juju/agents/unit-github-actions-exporter-0/charm/src/prometheus_alert_rules: directory does not exist"
This PR fixes it, update the library and add a temporary docs field to charmcraft run as expected.

Rationale

This way the charm can integrate with the Prometheus charm as expected.

Juju Events Changes

N/A

Module Changes

N/A

Library Changes

N/A

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Manging Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)

There is no src-docs for this charm.
There is no charmhub documentation for this charm.

[jdkandersson]

Please provide an explanation for anything that is not checked on the checklist

[github-actions]

Test coverage for 00eb40d

Name                             Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------------------
src/charm.py                        53      3      4      1    93%   98-100
src/charm_state.py                  36      1      8      1    95%   21
src/constants.py                     5      0      0      0   100%
src/exceptions.py                    4      0      0      0   100%
src/github_actions_exporter.py      24      0      0      0   100%
----------------------------------------------------------------------------
TOTAL                              122      4     12      2    96%

Static code analysis report

Run started:2023-09-15 13:44:59.441290

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 587
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):