fix: Prevent double root path when reporting metrics from FastAPI

[shaiarmis]

In FastAPI, when the root_path is configured, it's included both in the base_url and in the path:

root_path = "/api/v1"
base_url  = "https://acme.com/api/v1/"
path      = "/api/v1/my/endpoint"

To avoid reporting https://acme.com/api/v1/api/v1/my_endpoint, this PR adds trimming of the redundant root_path.

🧰 Changes

1. When constructing a URL, check if the request has root_path
2. If it does, and the path indeed starts with it, trim it from the path.

🧬 QA & Testing

Any FastAPI setup with the root_path configured.