Add Prometheus Receiver test for label_name_length_limit configuration

[mustafain117]

Description:
This PR depends on #6150 , commit 68e77b6 adds the test for label_name_length_limit
Adds TestLabelNameLimitConfig to test label_name_length_limit configuration for Prometheus receiver

TestLabelNameLimitConfig:
Sets the label_name_length_limit for Prometheus receiver
Tests the following two scenarios:

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

Link to tracking Issue:
PR 2/3 for #5997 . Following PR will add test for label_value_length_limit

[alolita]

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