Add egress tracking directly in mcp_root

[clockwork-tien]

Description of Changes

Based on #5775
Closes #5789

• Add egress tracking directly in mcp_root
  • Implement counting egress bytes for calls that go through mcp_root (host-wide MCP route), since egress middleware resolves database from the URL path, while mcp_root names its database in a tool argument inside the request body which path based middleware can not see.

API and ABI breaking changes

None.

Expected complexity level and risk

2

Testing

• I tested the implementation, tests, lint and format checks passed