Report Python's spans through the Rust reporter

One emitter now sends both Python's and Rust's spans to the Jaeger
agent. `init_tracer` sets the tracer up as before, with
`initialize_tracer` (its sampler, remote or not, its throttler, its
logging reporter, and the first-homeserver-wins guard), then swaps
jaeger-client's reporter for one whose `report_span` converts the
finished span into the Rust reporter's form, with the GIL held (a few
attribute reads), and queues it. The encoding and the UDP send happen
on the reporter thread, as for Rust's spans. Support for the external
`rust_python_jaeger_reporter` package is removed. If tracing in Rust
cannot start, jaeger-client's reporter stays.

The conversion follows jaeger-client's `make_jaeger_batch`, so the
datagrams are what jaeger-client sent, trace IDs from a remote context
wider than 128 bits included: a test encodes spans of every shape (all
tag types, logs, follows-from references, IDs at or above 2^63) both
ways and compares the bytes. A failure to convert is logged with its
traceback once, then at debug level.

One jaeger `PrometheusMetricsFactory` serves the process, as each
registers its metrics globally. jaeger-client's `reporter_*` options
and metrics no longer apply: the docs say what the Rust reporter does
instead. Tests run `init_tracer` and check that a Python span and a
Rust span under it arrive in one datagram, and that `logging: true`
keeps jaeger-client's logging reporter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W4zxygfJC93SsugxskhoJG

docs/opentracing.md

@@ -108,6 +108,16 @@ configure. A Rust span with a Python parent follows the parent's sampling
 decision. A trace that starts in Rust is sampled according to the `const` or
 `probabilistic` sampler. Under any other sampler, such traces are not sampled.
 
+The same reporter also sends the Python spans, in place of jaeger-client's own
+reporter and of the optional `rust_python_jaeger_reporter` package, which is no
+longer used. The rest of `jaeger_config` (the sampler, remote or not, the
+throttler, `logging`) applies as before. Its `reporter_queue_size`,
+`reporter_batch_size` and `reporter_flush_interval` options and jaeger-client's
+`jaeger:reporter_*` metrics no longer apply: the Rust reporter queues up to
+1000 spans, sends a batch at 100 spans, at 65000 bytes or after a second, and
+exports `synapse_rust_tracing_*` metrics instead. If tracing in Rust cannot
+start, jaeger-client's reporter sends the Python spans.
+
 The Rust reporter's counters are exported as Prometheus metrics named
 `synapse_rust_tracing_*`. A growing `synapse_rust_tracing_spans_dropped_total`
 means spans are finishing faster than they can be sent.