OTEL: fix end-to-end MCP trace interop

[strawgate]

FastMCP's OpenTelemetry support worked best when FastMCP owned the entire MCP exchange. In mixed setups, native FastMCP spans could compete with outer instrumentation for ownership of the MCP trace, server spans could prefer ambient transport context over the propagated MCP parent, and the FastMCP client still sent initialize and discovery requests like an opaque client. The result was traces with the right raw events but the wrong hierarchy at the MCP boundary.

This makes FastMCP compose cleanly as both client and server. FastMCP now has a supported propagation_only mode plus suppress_fastmcp_telemetry() for cases where another instrumentation layer should own MCP spans, server spans parent from propagated MCP context while linking ambient transport spans, baggage stays current during execution, and the FastMCP client now propagates _meta on initialize, tools/list, resources/list, resources/templates/list, and prompts/list. The new interop tests cover mixed instrumentation, opaque third-party MCP clients, and the missing client handshake/discovery paths so this behavior is exercised end to end instead of inferred.

from fastmcp import Client
from fastmcp.telemetry import suppress_fastmcp_telemetry

with tracer.start_as_current_span("execute_tool weather"):
    async with Client(server, auto_initialize=False) as client:
        with suppress_fastmcp_telemetry():
            await client.initialize()
            await client.list_tools()
            result = await client.call_tool("weather", {"city": "Chicago"})

Refs #3451. Closes #3993. Closes #3998.

🤖 Generated with Codex

[marvin-context-protocol]

CI failed due to two issues in tests/telemetry/test_interop.py: an import ordering violation (auto-fixed by ruff check) and a line that needs formatting (auto-fixed by ruff format). Run uv run prek run --all-files locally and push the result.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve extracted request context when telemetry is suppressed

When FASTMCP_TELEMETRY_MODE=propagation_only (or suppress_fastmcp_telemetry() is active), this early return skips _get_parent_trace_context() and never attaches the propagated MCP _meta context to the current execution context. In practice, server handlers then lose incoming trace/baggage (for example baggage.get_baggage(...) returns None) and any downstream inject_trace_context() call propagates the wrong context, which breaks the commit’s stated “propagation-only” interoperability contract for mixed/third-party clients.

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[strawgate]

Closing in favor of PR #4046 which has a clean single-commit history.