[Prometheus Receiver] Add test cases to validate honor_timestamp

[PaurushGarg]

Description: Added new Prometheus Receiver test case to validate honor_timestamp functionality.
TestHonorTimeStampsWithTrue validates:

• For initial scrape, Start_timestamp is same as point_timestamp, read from the explicit timestamp given to the datapoint in the testdata.
• Start_timestamp should be the explicit timstamp of first-time you see a particular metric
• Start_timetsamp should get reset if the current scrape has lower value than the previous scrape
• Currently, Prometheus scrape loop does not emit staleness markers for metrics of failed scrapes, and that's why histogram and summary start_timestamp exhibit no issues in this test.

TestHonorTimeStampsWithFalse validates:

• With honor_timestamp config set to false, valid testdata provided with explicit timestamps does not get honoured.

Link to tracking Issue:
Issue #6085

cc @alolita @Aneurysm9