Fix external plugin OpenTelemetry instrumentation

[JonasKunz]

What does this PR do?

Fixes #3145 .

The GlobalOpenTelemetry instrumentation was broken for external plugins due to a sideeffect of hiding the internal Otel-API shipped within the agent classloader from the IndyPluginClassloader:

• External Plugin IndyPluginClassloaders contain a copy of all classes of the external plugin jar. This means also GlobalOpenTelemetry is copied to the IndyPluginClassloader.
• Our instrumentation for GlobalOpenTelemetry has a classloader check ensuring that the target classloader can actually load the Otel-API. This is done by trying to load the io.opentelemetry.context.propagation.TextMapSetter.class resource
• IndyPluginClassloaders cannot locate the resources for their corresponding classes and therefore delegate this search to their parent classloader.
• The parent classloader filters out classes and resources which are supposed to be hidden from the plugin. With Added OpenTelemetry Metrics API bridge and instrumentation #3014 otel classes were added to this list.
• For this reason the instrumentation thinks that the copied GlobalOpenTelemetry is not instrumentable.

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

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• Start Time: 2023-06-05T23:39:18.235+0000

• Duration: 15 min 52 sec

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