fix(traces): update payload size after dropping traces

[lym953]

This PR

After dropping sampled-out traces, recompute body_size. This ensures the TraceAggregator's 3.2 MB batch limit is evaluated against only the data that will actually be sent to the backend.

Upgrades the version of some external crates because those are the versions used in libdatadog.

Testing

• Unit tests

Note

The recomputed size uses the protobuf encoded payload size, different from the original size, computed from: (from Claude)

• Datadog tracer path: src/traces/trace_agent.rs:506 — body_size is the first element of the tuple returned by get_traces_from_request_body() / get_v05_traces_from_request_body(), which is the raw HTTP request
  body byte count (msgpack-encoded bytes).
• OTLP path: src/otlp/agent.rs:167 — let body_size = size_of_val(&traces) where traces is a TracerPayloadCollection, giving only the stack size of the Rust enum (~32 bytes).

The new approach be more accurate because it's computed at a later point for the size of protobuf data, which is what's finally sent to Datadog.

By the way, the size doesn't need to be accurate. An approximate size is good enough. See:
https://github.com/DataDog/libdatadog/blob/3f3efefb2ff45d7a5491b770480396d001b87631/libdd-trace-utils/src/send_data/mod.rs#L148

Also here are some sample logs (logging node not included in the PR) showing the different sizes computed in different ways:

body_size (original): 582, body_size (encoded_len): 1355
body_size (original): 373, body_size (encoded_len): 1269
body_size (original): 336, body_size (encoded_len): 899