[exporter/prometheusremotewrite] only append colliding attributes values if they are different

[avanish-vaghela]

Description

While translating two OTel attributes into Prometheus Labels, if both attributes results in the same Prometheus label and both have the same value, they won't be appended.

Example:

• "bar.one": "foo"
• "bar/one": "foo"

The above two attributes when translated to Prometheus label would become

• Before the PR
  • "bar_one": "foo;foo"
• After the PR:
  • "bar_one": "foo"

Link to tracking issue

Fixes #35896

Testing

• Unit test added

Documentation

• Change log entry added