Add Prometheus Receiver test to verify time series with empty label values are supported

[mustafain117]

Description:
This PR adds a test (TestEmptyLabelValues) to verify the Prometheus Receiver supports time series with empty label values.
According to Prometheus documentation, a label with an empty label value is considered equivalent to a label that does not exist.

The test validates the following two scenarios:
Case 1: Metric data with one time series with an empty label value:

# HELP test_gauge0 This is my gauge
# TYPE test_gauge0 gauge
test_gauge0{id="1",testLabel=""} 19

Expected Result: testLabel is dropped

Case 2: Metric data with two time series with the same metric name, one has a value for testLabel and the other has an empty label value for testLabel:

# HELP test_gauge0 This is my gauge.
# TYPE test_gauge0 gauge
test_gauge0{id="1",testLabel=""} 19
test_gauge0{id="2",testLabel="foobar"} 2

Expected Result:

• testLabel is dropped for the first time series (test_gauge0{id="1",testLabel=""} 19)
• testLabel has value foobar for the second time series (test_gauge0{id="2",testLabel="foobar"} 2)

Link to tracking Issue:
Closes #5996

[mustafain117]

@dashpole @vishiy can you please provide a review for this PR? Thank you.

[alolita]

@bogdandrutu can you please merge? We are blocked on other PRs dependent on this one :-) Ty!