Python: Add OTEL trace headers on requests to MCP HTTP Streamable Tools

[tonybaloney]

The way the MCP HTTP streamable class is written, the headers are set at instantiation, which encourages folks to set headers once outside of the workflow.

For OTEL's tracing to work between AF and the MCP servers, we want to propagate the trace and span ID to the MCP servers in the calls.

This PR adds another header to MCP HTTP calls (disabled by default) with the standard OTEL headers. I've tested this on FastMCP with the starlette instrumentation and the traces to the MCPs are propagated correctly.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_mcp.py	327	115	64%	133, 143–144, 165, 190, 205, 211, 215, 217, 299, 317–322, 325–326, 336–338, 341–343, 345–351, 353–355, 358–359, 381–382, 386–391, 397–398, 402–403, 407, 409, 413–414, 418, 436, 456–466, 477, 490, 493–494, 498, 522, 525–526, 530, 561, 580, 582, 589–590, 609, 611, 617–620, 637–641, 760, 765–769, 877, 880–892, 894, 984, 987–989
TOTAL	11123	1785	83%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
1352	98 💤	0 ❌	0 🔥	28.121s ⏱️

[tonybaloney]

@moonbox3 please could you take a look at this idea

[moonbox3]

I think it’s better to build this around per-request dynamic headers, not mutate the tool's static dict. Capture the static headers once in MCPStreamableHTTPTool, then pass an optional dynamic_headers callable down into streamablehttp_client / StreamableHTTPTransport. The callable can pull the current span, make sure the context is valid, derive the sampled flag, and return {"traceparent": ...} only when it should be sent. We can then have _prepare_request_headers merge that callable's output on every POST so each RPC carries the active span while the static headers stay untouched. That then handles a possible KeyError when no headers were supplied and keeps the trace flag accurate instead of hardcoding 01.