[Python] How to add metrics to external services using aioprometheus and FastAPI?

I'm trying to add metrics to external services with aioprometheus in a FastAPI app. Here is a simplified example of what I'm trying to achieve.

Let's say, I have a wrapper App class as such:

from aioprometheus import Registry, Counter, Histogram
from fastapi import FastAPI

class App:
def __init__(self, ...):
self.registry = Registry()
self.counter = Counter(
name="counts", doc="request counts"
)
self.latency = Histogram(
name="latency",
doc="request latency",
buckets=[0.1, 0.5, 1, 1.5, 2]
)

self.app = FastAPI()
self._metrics()

def _metrics(self):
# Counter metrics
@self.app.middleware("http")
async def counter_metrics(request, call_next):
response = await call_next(request)
self.counter.inc(
{"path": str(request.url.path), "status": response.status_code}
)
return response

# Latency metrics
@self.app.middleware("http")
async def latency_metrics(request, call_next):
start = time.time()
response = await call_next(request)
total_duration = time.time() - start
self.latency.observe(
{"path": str(request.url.path)}, total_duration
)
return response

@self.app.on_event("startup")
async def startup():
self.app.include_router(some_router(...))

self.registry.register(self.counter)
self.registry.register(self.latency)


Basically, I have Registry, Counter, and Histogram initiated. In _metrics, I have Counter and Histogram specific logics that are later added to Registry. This will do its magic and catch the metrics when an endpoint in some_router is called (I would want to keep this, as well as have the external service metrics).

However, let's say, I call an external service from some_router as such:

from fastapi import APIRouter

def some_router():
router = APIRouter()

@router.get("/some_router")
async def some_router():
response = await external_service()

return response

return router


In this case, how would I add metrics specifically to external_service, i.e., Latency of this specific external service?

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