feat(POC): otelcol httpcheck receiver - use tags as metrics labels

[mfiedorowicz]

To allow using tags in the httpcheck receiver targets as metrics labels and be able to query metrics by common tags.

Example target definition:

{
  "endpoint": "https://orb.community",
  "method": "GET",
  "tags": {
    "tag_a": "valueA",
    "tab_b": "valueB"
}

Main issue with this is if the policy having httpcheck target with tags is being used with older orb agent not having policy sanitization, it will end up with validation error on otelcol and ending up policy not being applied.

[codecov-commenter]

Codecov Report

Attention: 80 lines in your changes are missing coverage. Please review.

Comparison is base (0a20a09) 48.54% compared to head (8dec896) 4.73%.

Files	Patch %	Lines
agent/otel/otlpmqttexporter/otlp.go	8.19%	56 Missing ⚠️
agent/backend/otel/otel.go	0.00%	11 Missing ⚠️
agent/backend/otel/policy.go	57.69%	11 Missing ⚠️
agent/otel/otlpmqttexporter/factory.go	33.33%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #2881       +/-   ##
===========================================
- Coverage    48.54%   4.73%   -43.81%     
===========================================
  Files           85      15       -70     
  Lines         6339    1141     -5198     
===========================================
- Hits          3077      54     -3023     
+ Misses        3262    1087     -2175     

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