Add River RPC span classification attributes

[daweifeng-replit]

Why

River spans already identify procedure type through river.method.kind, but the attribute is easy to miss when building Datadog queries and does not include standard RPC semantic fields. Adding explicit RPC classification attributes makes it easier to distinguish unary calls from subscriptions and streams in trace analysis and span-based metrics.

What changed

• Added shared procedure span attributes for both client and server River spans.
• Preserved the existing river.method.* attributes for compatibility.
• Added rpc.system, rpc.service, and rpc.method semantic attributes.
• Added river.rpc.kind with the River procedure kind: rpc, upload, subscription, or stream.
• Added river.rpc.streaming so callers can filter streaming procedures with one low-cardinality boolean.
• Covered client and server span attributes in tracing tests.

Test plan

Verified by CI (no reviewer action needed):

• npm run test:single -- tracing/tracing.test.ts passes locally and validates the emitted client/server span attributes.

For the reviewer to verify:

• Confirm the new attribute names work for Datadog span queries and future span-based metrics.

Versioning

• Breaking protocol change
• Breaking ts/js API change

Revertibility

Safe to revert. This only adds span attributes and a test; it does not change protocol behavior or public TypeScript APIs.

~ written by Zerg 👾 (mutated-zealot-0503)