exporter: Clarify headers formatting

[pellared]

What are you trying to achieve?

Clarify https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specifying-headers-via-environment-variables:

The OTEL_EXPORTER_OTLP_HEADERS, OTEL_EXPORTER_OTLP_TRACES_HEADERS, OTEL_EXPORTER_OTLP_METRICS_HEADERS environment variables will contain a list of key value pairs, and these are expected to be represented in a format matching to the W3C Correlation-Context, except that additional semi-colon delimited metadata is not supported, i.e.: key1=value1,key2=value2. All attribute values MUST be considered strings.

What did you expect to see?

Clear definition on the formatting.

Referring to quoting "W3C Correlation-Context" is bad because:

1. it does not refer to any stable specification (it even points to a master branch not even e.g. https://www.w3.org/TR/baggage/#baggage-http-header-format)
2. the name seems bad

Maybe it should be a format like a value of tracestate from the Trace Context W3C specification? See: https://www.w3.org/TR/trace-context/#tracestate-header-field-values?

Or should it be https://www.w3.org/TR/baggage/#header-content?

[pellared]

For reference, in open-telemetry/opentelemetry-go#4695 I decided to reference https://www.w3.org/TR/baggage/#header-content.

This is also consistent with https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/sdk.md#specifying-resource-information-via-an-environment-variable.

I propose that in both places we would refer to the "working draft" version: https://www.w3.org/TR/baggage/#header-content