Clarify that OTLP exporter options can be set as code

[carlosalberto]

Currently specification/protocol/exporter.md is de facto about how the configuration options must be exposed as env vars, but we should add a super-clear note that is done in addition to they exposed via code too.

[jack-berg]

I think this is covered by this language:

The SDK MUST provide a programmatic interface for all configuration. This interface SHOULD be written in the language of the SDK itself. All other configuration mechanisms SHOULD be built on top of this interface.

To your point, it could be helpful to reinforce this in the specification/protocol/exporter.md document.

[tigrannajaryan]

@carlosalberto closing since it already seems to be covered. Please reopen if needed.