[Feature] Add metrics and a built-in grafana dashboard #634

Is your feature request related to a problem? Please describe.

Hi Team , I will try to add metrics such as the number of active sessions and tool call success rate, etc... And then build an official built-in grafana dashboard to provide better observability for fastmcp.

Describe the solution you'd like

Added two extension points and provided a default implementation based on opentelemetry.

1. request-level metrics and tracing through middleware
2. tool-level callback extension for metrics statistics.