[FEATURE]: propagate W3C trace context on outbound MCP HTTP/SSE requests #28295

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Describe the enhancement you want to request

Problem

When opencode connects to an MCP server over HTTP, the outbound requests do not carry the W3C traceparent / tracestate headers.

Expected Behavior

With OTel tracing enabled (OTEL_EXPORTER_OTLP_ENDPOINT set and experimental.openTelemetry: true), every outbound MCP HTTP request should carry the W3C traceparent (and tracestate when present) header derived from the currently-active trace context, so the MCP server's spans attach as children of the originating opencode span in the same trace.

Use Case

Operating a tracing-enabled MCP server (whether self-hosted or shared) and needing a single, complete trace per tool call across all hops.

Concretely, this unblocks:

• Debugging tool-call latency end-to-end (where time is spent on the opencode side vs. inside the MCP server vs. in whatever the MCP server calls downstream).
• Correlating errors raised inside the MCP server with the originating opencode session in a single view.
• Consistent sampling

Related Issues

• Add OTEL export capabilities for observability and monitoring #12142 — the umbrella OTel export work this builds on.
• [FEATURE]: propagate incoming trace context for HTTP requests #18801 — the inbound mirror of this gap on opencode's own HTTP server.
• [FEATURE]: propagate trace context to subprocesses #27031 — the same correlation gap for child processes.