Add sdk-config.yaml starter template w/ references to env vars

[jack-berg]

Part of the configuration working group recommendation on how to move forward with #3752 as described here.

• Part of Define OTEL_EXPERIMENTAL_CONFIG_FILE to ignore other env vars, add env var substitution default syntax opentelemetry-specification#3948
• Adds a new sdk-migration-config.yaml to the examples directory, which includes env var substitution references to all env vars which map cleanly to file config
• Adds a new sdk-config.yaml to the example directory, which is identical to sdk-migration-config.yaml, but without env var substitution references
• Adds a new step to the makefile build which performs env var substitution using js envsub which allows validation to take place after substitution
• Updates the schema so all types can be null. This ensures a config file is still valid after performing env var substitution with an undefined env var.

[marcalff]

LGTM, with fixes on endpoints.

[marcalff]

Suggested change

	endpoint: ${OTEL_EXPORTER_OTLP_PROTOCOL:-http://localhost:-4318}
	endpoint: ${OTEL_EXPORTER_OTLP_PROTOCOL:-http://localhost:4318/v1/traces}

[tsloughter]

That would make it OTEL_EXPORTER_OTLP_TRACES_ENDPOINT

Also, its currently using OTEL_EXPORTER_OTLP_PROTOCOL when I'm guessing it wants either OTEL_EXPORTER_OTLP_TRACES_ENDPOINT or to actually put the path outside the environment variable default to apply to both:

endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:-http://localhost:4318}/v1/traces

[marcalff]

Good point on the TRACES part. So, something like this then:

Suggested change

	endpoint: ${OTEL_EXPORTER_OTLP_PROTOCOL:-http://localhost:-4318}
	endpoint: ${OTEL_EXPORTER_OTLP_TRACES_ENDPOINT:-http://localhost:4318/v1/traces}

This also means using the TRACES env var for PROTOCOL, CERTIFICATES, CLIENT_KEY, etc as well.

[jack-berg]

We have to decide between the generic version of these properties and the signal specific. I think the generic version of these is likely to be more popular. But the generic version of the property doesn't require you to configure a path.

One thing which is still ambiguous is whether the endpoints specified in file config have paths appended automatically if the protocol is http/protobuf and the path is omitted. I lean towards yes.

If the answer is no, then we should go back and update the other examples, which currently reference http://localhost:4318 without any path: https://github.com/open-telemetry/opentelemetry-configuration/blob/main/examples/kitchen-sink.yaml#L64

[marcalff]

Suggested change

	endpoint: ${OTEL_EXPORTER_OTLP_PROTOCOL:-http://localhost:-4318}
	endpoint: ${OTEL_EXPORTER_OTLP_METRICS_ENDPOINT:-http://localhost:4318/v1/metrics}

[marcalff]

Suggested change

	endpoint: ${OTEL_EXPORTER_OTLP_PROTOCOL:-http://localhost:-4318}
	endpoint: ${OTEL_EXPORTER_OTLP_LOGS_ENDPOINT:-http://localhost:4318/v1/logs}

[jack-berg]

This is a loose end from open-telemetry/opentelemetry-specification#3948

I've pushed an additional commit and marked as ready for review. Please review when you have the chance.

cc @open-telemetry/configuration-maintainers