Clarify if OTEL_EXPORTER_OTLP_SPAN_INSECURE and OTEL_EXPORTER_OTLP_METRIC_INSECURE are required for new implementation

[pellared]

What are you trying to achieve?

I want to stabilize the OTel Go OTLP metrics exporter.

opentelemetry-specification/specification/protocol/exporter.md

Lines 59 to 62 in 3a1dde9

	**[2]**: The environment variables `OTEL_EXPORTER_OTLP_SPAN_INSECURE`
	and `OTEL_EXPORTER_OTLP_METRIC_INSECURE` are obsolete because they do not follow
	the common naming scheme of the other environment variables. They are still
	supported because they were part of a stable release of the specification.

What is the intention behind using the term 'obsolete' in this context when the specification explicitly mandates support for both environment variables?

opentelemetry-specification/specification/protocol/exporter.md

Line 64 in 3a1dde9

Use `OTEL_EXPORTER_OTLP_TRACES_INSECURE` instead of `OTEL_EXPORTER_OTLP_SPAN_INSECURE` and `OTEL_EXPORTER_OTLP_METRICS_INSECURE` instead of `OTEL_EXPORTER_OTLP_METRIC_INSECURE`.

What is the purpose of including this line if the specification dictates the continued support of the old environment variable?
Is this line primarily for user information? Is there any concern in using e.g. OTEL_EXPORTER_OTLP_SPAN_INSECURE if it remains stable and supported? Are there any intentions to discontinue or remove it in the future?

Additional context.

Based on the information provided in #2240, it appears that the author (@tsloughter) intended to convey that OTEL_EXPORTER_OTLP_SPAN_INSECURE and OTEL_EXPORTER_OTLP_METRIC_INSECURE are older environment variables, and if they were previously implemented, they should continue to be supported. It's implied that new implementations may not necessarily need to support OTEL_EXPORTER_OTLP_SPAN_INSECURE and OTEL_EXPORTER_OTLP_METRIC_INSECURE.

If that's the case, I believe we should aim for clearer wording. However, would this be regarded as an unacceptable breaking specification change or an acceptable fix? See #3719