[Prometheus Receiver] Add test to validate ‘Untyped’ Metric type

[PaurushGarg]

Is your feature request related to a problem? Please describe.
In order to verify that the Prometheus receiver is functioning as expected for Prometheus to OTLP data transformations, a test is required for validating all Prometheus supported metric types - specifically Gauge, Counter, Summary, Histogram, and Untyped. The current Prometheus receiver test do not check for ‘Untyped’ metric type where the # TYPE is ‘unknown’. A test for 'Untyped' metric type is needed to be added.

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

Add an additional test case which exposes Untyped Prometheus metric type as data-set for the Prometheus receiver to scrape. This data-set should contain an 'Untyped' metric type where the # TYPE is ‘unknown’. This test could be similar to the existing test for other metric types (Gauge, Counter, Histogram, Summary) in metrics_receiver_test.go.

Expected behavior: An Untyped metric should work the same as a Gauge. The only difference is that no type information is implied. The ‘Untyped’ metric should be cast to a GAUGE_DOUBLE type by the Prometheus Receiver. This will ensure the test to pass.

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

Related Open Issues:
#4743
#5001
cc: @alolita @Aneurysm9

[PaurushGarg]

@alolita please assign this issue to me. I would like to work on this one.