Fix issue where Prometheus SD config cannot be parsed

[alvinlin123]

Description:

Fixes issue #1867, the issue itself has background of the bug.

Testing:

• I added unit tests to ensure all SD configs can be parsed.
• I also tested by building an executable and ensure it can load sd configs.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Alvin Lin (c1adee3, 25a5ba0)

[codecov]

Codecov Report

Merging #1877 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1877      +/-   ##
==========================================
- Coverage   91.29%   91.12%   -0.18%     
==========================================
  Files         273      263      -10     
  Lines       16365    16107     -258     
==========================================
- Hits        14940    14677     -263     
- Misses        998     1003       +5     
  Partials      427      427              

Impacted Files	Coverage Δ
receiver/prometheusreceiver/factory.go	78.78% <ø> (ø)
...mplingprocessor/tailsamplingprocessor/processor.go	66.66% <0.00%> (-7.38%)	⬇️
config/config.go	96.09% <0.00%> (-2.79%)	⬇️
translator/internaldata/oc_to_metrics.go	91.97% <0.00%> (-0.41%)	⬇️
processor/metrics.go	71.76% <0.00%> (ø)
service/telemetry.go	81.96% <0.00%> (ø)
consumer/pdata/trace.go	100.00% <0.00%> (ø)
consumer/pdata/generated_log.go	100.00% <0.00%> (ø)
consumer/pdata/generated_trace.go	100.00% <0.00%> (ø)
processor/queuedprocessor/factory.go	100.00% <0.00%> (ø)
... and 25 more

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[ocervell]

@bogdandrutu can we release a bug fix release for this ?
Currently we cannot use the Datadog exporter (introduced after the Prom receiver was broken) and Prometheus receiver at the same time ...

The latest version runs into this Prom bug, and the version before doesn't have the Datadog exporter implemented.