Use HTTP tracer if env var is set

[trajano]

Fixes #5743

Tested via example from https://github.com/trajano/trajano-portfolio/blob/master/Dockerfile#L17-L24

FROM caddy:builder AS builder
RUN git clone https://github.com/trajano/caddy /mnt/
RUN xcaddy build \
  --with github.com/caddyserver/caddy/v2=github.com/trajano/caddy/v2@ea6a6298f5ef94efef2900af22d200583ca09f3f

FROM caddy:alpine
COPY --from=builder /usr/bin/caddy /usr/bin/caddy

Using Grafana.com to receive the OTEL traces.

This would require a change to the tracing documentation https://caddyserver.com/docs/caddyfile/directives/tracing#tracing

Enables integration with OpenTelemetry tracing facilities, using [opentelemetry-go](https://github.com/open-telemetry/opentelemetry-go) and [opentelemetry-go-contrib](https://github.com/open-telemetry/opentelemetry-go-contrib) .

When enabled, it will propagate an existing trace context or initialize a new one.

It uses the exporter protocol specified by `OTEL_EXPORTER_OTLP_PROTOCOL` and the uses [gRPC](https://github.com/grpc/) as the default.  In addition it uses W3C [tracecontext](https://www.w3.org/TR/trace-context/) and [baggage](https://www.w3.org/TR/baggage/) as propagators.

The trace ID is added to [access logs](https://caddyserver.com/docs/caddyfile/directives/log) as the standard traceID field.  The trace ID is also set as the `http.vars.trace_id` placeholder value.

[CLAassistant]

All committers have signed the CLA.

[trajano]

Added another commit primarily to offload the work to OTEL's autoexport function.

Hopefully this will eventually allow us to send logs to OTEL as well.

[trajano]

Had to revert that last commit. It didn't seem to work as advertised.

[trajano]

Un reverting the revert. My test was actually flawed and it was working

[trajano]

Note this will just get it to function the same way as before. There's a lot missing in Caddy's OTEL messages. To name a fiew

• there's no Span.Kind pieces for the Client portion so there's no offload shown in the traces from Caddy to the reverse proxy. Add Otel Trace client interactions #6487 will add that
• Status should be ok not just left as unset when it's the final response.
• The foward_auth appears as a separate trace
• The OTEL exports only supports tracing but not others like logs or metrics