Better defaults for OTEL tracing.

The doc offers the following command to run quickwit and make it possible to connect jaeger to it.

QW_ENABLE_OTLP_ENDPOINT=true \ # enables the OTLP endpoint. It is actually already true by default
QW_ENABLE_OPENTELEMETRY_OTLP_EXPORTER=true \ # Enables exporting the OTLP info
OTEL_EXPORTER_OTLP_ENDPOINT=http://127.0.0.1:7281 \ # Endpoint to which we should ship the OTEL information... 
quickwit run

On single node, it should be the default and ./quickwit run should be sufficient to reach the point where
quickwit indexes its own logs.

But we also still want to cater to the following use cases in a reasonably clear manner and in that order:

• user wants to export quickwit tracing to the OTEL collector, using the standard env variable OTEL_EXPORTER_OTLP_ENDPOINT
• user is running quickwit in a distributed fashion, and may not be running the indexer on this machine.

Proposed dream solution

We have an environment variable called
QW_TRACING=
It takes the value "quickwit", "otel", or "disabled". (default: "quickwit")

If set to quickwit, quickwit forwards the traces to a quickwit node indexing traces.
If set to otel, the logs are sent to the grpc OTEL endpoint defined via OTEL_EXPORTER_OTLP_ENDPOINT

The otel ingest endpoint is enabled by default on all servers, and on server that do not host the pipeline, the requests are forwarded.