Allow count_exceptions to add the error type as a label

從 Counter.count_exceptions 和 context_managers.ExceptionCounter 開始，接著追蹤 with 方法如何處理例外類型和標籤。定義如何在保留現有呼叫和行為的同時新增錯誤標籤，並透過向後相容性測試驗證所要求的 endpoint 和例外標籤使用情境。

I'd like count_exceptions to add the error type as a label so I can track what errors are occurring against what endpoints for my application.

I'd like to do this via the with method.

from prometheus_client import Counter

ERRORS = Counter(
    'http_request_errors_count',
    'Count of  exceptions that happen during an HTTP request',
    ['method', 'endpoint', 'error']
)

async def error_middleware(request, handler):
    try:
        with ERRORS.count_exceptions(request.method, request.path):
            return await handler(request)
    except SomeHTTPException as e:
        ...

To do this I've created my own custom Counter classes.

from prometheus_client import Counter, context_managers


class CustomExceptionCounter(context_managers.ExceptionCounter):
    def __init__(self, counter, exception, *labels):
        self._counter = counter
        self._exception = exception
        self._labels = labels

    def __exit__(self, typ, value, traceback):
        if isinstance(value, self._exception):
            self._counter.labels(*(*self._labels, typ.__name__)).inc()  # Append exception's name as a label


class CustomCounter(Counter):
    def count_exceptions(self, *labels, exception=Exception):
        return CustomExceptionCounter(self, exception, *labels)


ERRORS = CustomCounter(
    'http_request_errors_count',
    'Count of CancelledError exceptions',
    ['method', 'endpoint', 'error']
)

I think there is a general solution that's backward compatible, I've just not figured it out. Is this something you'd support? I'd be happy to submit a PR if I can figure out how to make this backward compatible.