Add OTEL_EXPORTER_OTLP_PROTOCOL env variable support

[akvlad]

In the opentelemetry documentation you have link to, there is a configuration of the protocol for traces to send: https://github.com/open-telemetry/opentelemetry-specification/blob/v1.7.0/specification/protocol/exporter.md#specify-protocol. It can be grpc or http/protobuf etc.

As far as I see in the code: https://github.com/caddyserver/caddy/blob/master/modules/caddyhttp/tracing/tracer.go#L61 you have only grpc supported.

It would be great to have other options like http/protobuf supported as well. Some tracing ingesters have the no GRPC support by different reasons.

[francislavoie]

PRs welcome. Most maintainers don't use tracing or metrics ourselves, so we need to lean on the community to contribute improvements.