[Prometheus Receiver] Added testcase for core Prometheus metrics

[PaurushGarg]

Description:

• Added testcase for Prometheus core metrics to cover the start_timestamp reset issue.
• Modified isFirstFailedScrape method to validate first failed scrape on the basis of datapoint flags supported by OTLP format for staleness markers. For now skipped this validation with comment, as current Prometheus Receiver metric builder is not currently setting flags.
• Modified testComponent method to accept custom receiver configs. This is required to avoid multiple copies of this function (because of custom configs).
• Modified other tests to remove testComponent copy and use method testComponent in metric_receiver_helper_test.go

Link to tracking Issue:
Issue Number: #6000
cc @alolita @Aneurysm9