Add Prometheus exporter to ROCK

[gtrkiller]

Applicable spec: ISD043

Overview

Add the NGINX Prometheus Exporter as a layer to the content-cache ROCK.

Rationale

Simplify the charm, reduce container amount, address possible securities issues caused by docker images.

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 - 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)

[amandahla]

LGTM

[github-actions]

Test coverage for 4f119cd

Name                 Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------
src/charm.py           186     11     39      8    92%   136-137, 157-158, 177, 218-220, 234->246, 318->321, 343, 356, 365->368, 443
src/file_reader.py      17      0      6      0   100%
----------------------------------------------------------------
TOTAL                  203     11     45      8    92%

Static code analysis report

Run started:2024-01-05 14:36:19.231684

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1257
  Total lines skipped (#nosec): 1
  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):

[amandahla]

LGTM