missing variables and doc for endpoint credentials

[travisghansen]

https://github.com/open-telemetry/opentelemetry-operator/blob/main/cmd/otel-allocator/README.md#service--pod-monitor-endpoint-credentials

The pre-reqs described in that doc are not sufficient. The feature was not working and eventually I noticed the following in the logs:

Cert-Manager is not available to the operator, skipping adding to scheme.

I do indeed have cert-manager installed so the log was a bit perplexing. After digging through the code I found this: https://github.com/open-telemetry/opentelemetry-operator/blob/main/internal/autodetect/autodetectutils/utils.go which detects the service account name and only does so via env var. This should probably be documented in the above link.

In addition it may be good to have more seamless integration with the helm chart to autoset the env vars.

Alternatively, it may be easier to have the code decode the token file and pull the data out similar to reading the data from the namespace file.

manager:
  env:
    # these are required to make cert-manager detection work properly
    SERVICE_ACCOUNT_NAME: {{ .Release.Name }}
    NAMESPACE: {{ .Release.Namespace }}