Add test for Job and Instance Labels for Prometheus receiver

[mustafain117]

Is your feature request related to a problem? Please describe.
Currently, the Prometheus receiver test for labels only checks for the presence of external labels. We want to validate the presence of job and instance labels in every timeseries.

Describe the solution you'd like
We propose the following solution:

Similar to the test for external labels

opentelemetry-collector-contrib/receiver/prometheusreceiver/metrics_reciever_external_labels_test.go

Line 37 in 46b047b

func TestExternalLabels(t *testing.T) {

Add a test for job and instance labels that will setup a static target for the Prometheus receiver to scrape, the metrics sent by the receiver to the Metrics sink can then be validated to assert that the job labels and instance labels are present in each scrape with the correct expected values.

Additional context
Related to : open-telemetry/wg-prometheus#57

CC: @alolita @Aneurysm9

[mustafain117]

@alolita please assign this issue to me.

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