fix: include _internal/mcp in observability tracing paths

[ecthelion77]

🐛 Bug-fix PR

🔗 Issue

Closes #4152

📌 Summary

The _internal/mcp endpoint path is not included in the observability tracing include patterns, causing internal MCP calls between gateway components to be invisible in distributed traces. This makes it impossible to trace the full request flow when meta-servers or virtual servers invoke internal MCP endpoints.

🔁 Reproduction Steps

1. Configure a meta-server that calls internal MCP endpoints via /_internal/mcp/
2. Send a request that triggers the meta-server
3. Check traces in your observability backend (Jaeger/Tempo)
4. Notice that the /_internal/mcp leg is missing from the trace

🐞 Root Cause

The OTEL_TRACE_INCLUDE_PATTERNS default in config.py lists explicit path patterns for tracing (/servers/*/sse, /servers/*/message, /a2a), but /_internal/mcp was omitted.

💡 Fix Description

Add r"^/_internal/mcp(?:/|$)" to the default OTEL_TRACE_INCLUDE_PATTERNS list in Settings, so internal MCP calls are traced by default.

🧪 Verification

Check	Command	Status
Lint suite	make lint	✅
Unit tests	make test	✅
Coverage ≥ 80 %	make coverage	✅
Manual regression no longer fails	Verified traces include /_internal/mcp spans	✅

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed

[ecthelion77]

Suggested labels: bug, COULD, python, observability