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

[avanish-vaghela]

Description

While translating two OTel attributes into Prometheus Label, if two attributes being the same with the same value, this change avoids the situation of duplicate values separated by semicolon.

Example:

• "bar.one": "foo"
• "bar/one": "foo"

The above two attributes when translated to Prometheus label would become

• Before this change
  • "bar_one": "foo;foo"
• After the change
  • "bar_one": "foo"

Link to tracking issue

Fixes open-telemetry#35896

Testing

• unit test added

Documentation