utils: explicit OTEL export config and OTEL enablement via common entry point (#11139)

We want to export performance traces from the pageserver in OTEL format.
End goal is to see them in Grafana.

To this end, there are two changes here:
1. Update the `tracing-utils` crate to allow for explicitly specifying
the export configuration. Pageserver configuration is loaded from a file
on start-up. This allows us to use the same flow for export configs
there.
2. Update the `utils::logging::init` common entry point to set up OTEL
tracing infrastructure if requested. Note that an entirely different
tracing subscriber is used. This is to avoid interference with the
existing tracing set-up. For now, no service uses this functionality.

PR to plug this into the pageserver is
[here](https://github.com/neondatabase/neon/pull/11140).

Related https://github.com/neondatabase/neon/issues/9873

libs/utils/src/logging.rs

@@ -165,6 +165,7 @@ pub fn init(
         };
         log_layer.with_filter(rust_log_env_filter())
     });
+
     let r = r.with(
         TracingEventCountLayer(&TRACING_EVENT_COUNT_METRIC).with_filter(rust_log_env_filter()),
     );