[OTel]: Spin should support `OTEL_EXPORTER_OTLP_HEADERS`

[ThorstenHans]

Having support for OTEL_EXPORTER_OTLP_HEADERS could unlock several scenarios. For example, we could support OTLP exporters that require header based authentication.

Spin should allow users to specify individual headers that will be used when emitting traces, logs, or metrics.

If the OTEL_EXPORTER_OTLP_HEADERS environment variable is present, Spin should configure outgoing headers on both TonicExporterBuilder and HttpExporterBuilder using the with_headers function

(see https://docs.rs/opentelemetry-otlp/0.26.0/opentelemetry_otlp/struct.HttpExporterBuilder.html#method.with_headers for example)

More about OTEL_EXPORTER_OTLP_HEADERS:

https://opentelemetry.io/docs/languages/sdk-configuration/otlp-exporter/#otel_exporter_otlp_headers

[ThorstenHans]

This has already been requested by @endocrimes in #2816

[lann]

Have you tried setting this env var already? The otel library we use appears to already support it (from a very brief code review).

[endocrimes]

#2857 it looks like it should work now

[ThorstenHans]

yep. Can confirm that it works 👍🏻