Description
For requests to endpoints defined in FastAPI sub-applications, the mount path is chosen as the transaction name. I expected the full route of the endpoint.

To Reproduce

1. Run the following simple FastAPI app:

import uvicorn
from elasticapm.contrib.starlette import ElasticAPM, make_apm_client
from fastapi import FastAPI

app = FastAPI()
sub = FastAPI()
app.mount("/sub", sub)

apm = make_apm_client(
    {
        "SERVICE_NAME": "sub-app-test",
    }
)

app.add_middleware(ElasticAPM, client=apm)


@sub.get("/hi")
async def hi():
    return "hi"


@sub.get("/bye")
async def bye():
    return "bye"


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8888)

• Observed behavior
  The transactions of /sub/hi and /sub/bye are both named /sub and grouped.
  

• Expected behavior
  The transactions of /sub/hi and /sub/bye are named according to the full route.

Environment

• OS:
  • Client: Windows
  • Server: Ubuntu
• Python version: 3.7.3
• Framework and version: fastapi==0.61.2
• APM Server version: docker image elasticsearch/elasticsearch:7.10.2
• Agent version: elastic-apm==6.0.0