Add COS integration and metric logs

[yhaliaw]

Overview

• Add support for cos-agent integration. This will enable ingestion of logs on the charm to COS.
• Add logging of metrics of the nginx.

Rationale

The two changes will allow metrics to be ingested to COS for a dashboard to be built.

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing 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)

[github-actions]

license-eye has checked 78 files.

Valid	Invalid	Ignored	Fixed
37	2	39	0

Click to see the invalid file list

• content-cache/lib/charms/grafana_agent/v0/cos_agent.py
• lib/charms/grafana_agent/v0/cos_agent.py

Use this command to fix any missing license headers

```bash

docker run -it --rm -v $(pwd):/github/workspace apache/skywalking-eyes header fix

</details>

[alithethird]

Looks good, minor doc comments.

[jdkandersson]

For your next PR, please do the public interface first and get it reviewed by the allocated senior reviewer. You can read more in ISD062

[github-actions]

Test coverage for 6361378

Name                   Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------
src/certificates.py       21      2      6      2    85%   65-68
src/charm.py             101     12      8      0    89%   64-65, 120, 149, 153-156, 209-211, 222-224
src/errors.py             16      0      0      0   100%
src/nginx_manager.py     130     21     18      4    83%   142-144, 151, 154-160, 163, 201-203, 254-256, 272-276, 293->295
src/state.py             141      9     36      1    93%   353-363
src/utilities.py          13      0      0      0   100%
------------------------------------------------------------------
TOTAL                    422     44     68      7    89%

Static code analysis report

Run started:2024-12-10 03:16:20.370788

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 931
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 2

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):