[lesnik512]

Closes #451.

Changes

• Use the configured Prometheus registry for FastMCP HTTP metrics, tool metrics, and the scrape endpoint. The global registry remains the default.
• Keep the optional in-progress gauge in the configured registry and share it across generated HTTP apps with matching gauge settings.
• Document the custom-registry scrape in the 2.x migration guide.

Verification

• Reproduced the missing HTTP counts in a failing /metrics test. The passing test scrapes two HTTP requests across generated apps, a registered tool call and its duration count, and an application counter.
• Verified the in-progress gauge increments during requests, returns to zero, and honors per-app gauge names and labels.
• 669 tests passed with 100% line coverage. The 22 focused Prometheus tests passed at the dependency floors. Ruff, ty, and the strict docs build passed.