Add Prometheus Receiver test for label_limit configuration

[mustafain117]

Description:
Adds TestLabelLimitConfig to test label_limit configuration for Prometheus receiver

TestLabelLimitConfig:
Sets the label_limit for Prometheus receiver
Tests the following two scenarios:

• Positive test case: Prometheus receiver scrapes metrics which are within the label_limit
  • Assert scrape is successful
• Negative test case: Prometheus receiver scrapes metrics which exceed label_limit
  • Assert scrape is unsuccessful

Renamed metrics_reciever_external_labels_test.go to metrics_receiver_labels_test.go

Link to tracking Issue:
First of 3 PRs for issue #5997
Following 2 PRs will add tests for label_name_length_limit configuration & label_value_length_limit configuration

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[alolita]

@dashpole @vishiy can you provide a review? Then we should be ready to merge. Ty!!