Inject OTEL environment variables into containers

[ngehrsitz]

Is your feature request related to a problem? Please describe.
At the moment you need to manually set the OTEL_ environment variables to use OpenTelemetry SDK instrumentation in your application.
But the required information is already available to a container in /var/lib/dynatrace/enrichment/endpoint/endpoint.properties and /var/lib/dynatrace/enrichment/dt_metadata.properties if the annotations oneagent.dynatrace.com/inject and metadata-enrichment.dynatrace.com/inject are enabled.
This works by having the operator create the dynatrace-metadata-enrichment-endpoint secret and then mounting that as a file to the container. This means all of the required information is already present in the container, but requires additional code in the application to read this custom format.

Describe the solution you'd like
A better approach would be for the operator to create a secret with the credentials or add them to the existing secret. This would make it possible to inject the credentials into a container as the standardized OTEL_ environment variables. This feature should of course also be enabled separately using a feature flag.

Describe alternatives you've considered
The current alternative to this is to either manually create such a kubernetes secret or adapt your application to read the dynatrace format which is rather inconvenient.

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[ngehrsitz]

Continuing in https://community.dynatrace.com/t5/Product-ideas/dynatrace-operator-should-inject-OTEL-environment-variables-into/idi-p/264475