Previously init_observability() called init_json_logger() (which set the
global subscriber with a fmt layer) and then init_tracing() (which only
set the global tracer provider). Because no tracing_opentelemetry::layer()
was ever added to the subscriber, #[instrument] spans never reached Tempo.
Now init_observability() builds a single Registry subscriber that
composes an EnvFilter, a JSON fmt layer, and an optional OTLP layer
(via tracing_opentelemetry). The OTLP layer gracefully degrades to None
when Tempo is unavailable, so logging always works.
Also adds build_otel_tracer() to tracing_config.rs which returns
Option<Tracer> for the caller to wrap in an OpenTelemetryLayer inside
their subscriber stack, letting the S type parameter be inferred
correctly by the compiler.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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