docs(cloudflare): document Worker observability.traces support

[sam-goodwin]

Closes #1438

Background

Issue #1438 requested first-class support for observability.traces on Cloudflare.Worker. On investigation, the functional support already exists end-to-end:

• WorkerObservability.traces type (enabled, headSamplingRate, persist, destinations) is defined in worker.ts.
• It is serialized to the Cloudflare API via camelToSnakeObjectDeep in worker-metadata.ts.
• It round-trips back onto worker.observability, with a passing live test in worker.test.ts asserting both the API metadata and the output shape.

The real gap the issue describes — "explicit documentation/examples appears to be missing/partial" — was documentation, plus a small type incompleteness.

Changes

• worker-metadata.ts: added persist and destinations to the observability.logs metadata type. The runtime already forwarded these (and the existing test asserts them), but the type omitted them.
• worker.mdx: added a dedicated Observability section documenting the full enabled / logs / traces configuration, plus a Traces subsection with a Custom Spans (trace.enterSpan()) example and production-vs-dev sampling guidance. Also surfaced traces in the quick-config snippet.

Usage

export const worker = await Worker("api", {
  entrypoint: "./src/worker.ts",
  observability: {
    enabled: true,
    headSamplingRate: 1,
    logs: { enabled: true, invocationLogs: true, persist: true },
    traces: { enabled: true, headSamplingRate: 0.1, persist: true },
  },
});

And custom spans from within the Worker:

import { trace } from "cloudflare:workers";

export default {
  async fetch(request, env, ctx) {
    return trace.enterSpan("handle-request", async (span) => {
      span.setAttribute("route", new URL(request.url).pathname);
      const user = await env.DB.prepare("SELECT * FROM users LIMIT 1").first();
      return Response.json(user);
    });
  },
};

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