Enable to use a function for headers in exporters

[rmannibucau]

Is your feature request related to a problem? Please describe.

I need to send dynamic headers (think oauth2 token refresh even if not exactly my use case it is exactly the same kind of issue).

Describe the solution you'd like

The http transport takes a function returning headers but it is not exposed in the oltp exporter (trace one for me) which only supports a static map/record.

Idea is to just enable it to be a static record or a function returning a record.

Describe alternatives you've considered

Dropping and recreating the exporter and facading the dynamic impl but this is overkill.

[SebastiaanVanspauwen]

I am currently facing the same issue, using workaround mentioned in #2903 (works for @opentelemetry/exporter-trace-otlp-http 0.52.1), but the same metrics version (@opentelemetry/exporter-metrics-otlp-http 0.52.1) does not allow header overwriting