[Prometheus Receiver] Add test to validate StaleNaNs, NormalNaNs, and +-Inf are correctly passed through

[PaurushGarg]

Is your feature request related to a problem? Please describe.

In addition to standard numerical values, Nan, +Inf, and -Inf are valid values representing not a number, positive infinity, and negative infinity respectively.

In order to verify that the Prometheus receiver is functioning as expected for Prometheus to OTLP data transformations, a new test is needed to validate that the StaleNaN, NormalNaN, and +-Inf are correctly handled by Prometheus Receiver and is transformed to OTLP data.

Describe the solution you'd like
Add new following testcases:

• TestCase 1: Validate StaleNaN

Prometheus Receiver should assign metric a StaleNaN value, if the metric is missing in the current scrape but was present in the previous scrape. In order to validate the Prometheus Receiver for StaleNaN, provide testdata as described, and configure Prometheus Receiver to scrape this testdata.

Expected behavior: The metric with StaleNaN value should work the same way as metrics with standard numerical values. The only difference is that metric value of OTLP data should be StaleNaN. This can verified using IsStaleNaN(v float64) method.

• TestCase 2: Validate NormalNaN

• Create testdata with default metrics and their values set as NormalNaN.
  For Example:

var normalNaNPage1 = `
# HELP go_threads Number of OS threads created
# TYPE go_threads gauge
go_threads NaN

Expected behavior: The metric with NomralNaN value should work the same way as metrics with standard numerical values. The only difference is that metric value of OTLP data should be NormalNaN. This can verified using value.NormalNaN.

• TestCase 3: Validate +-Inf

• Create testdata with default metrics and their values set as Inf and -Inf.
  For Example: Testdata to validate Inf

var InfPage1 = `
# HELP go_threads Number of OS threads created
# TYPE go_threads gauge
go_threads +Inf

Expected behavior: The metric with +-Inf value should work the same way as metrics with standard numerical values. The only difference is that metric value of OTLP data should be +-Inf. This can verified using math.IsInf(f float64, sign int).

Additional context
Related to open-telemetry/prometheus-interoperability-spec#57
cc: @alolita @Aneurysm9

[PaurushGarg]

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

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