Create a `GrpcApiClient` base class

[llucax]

What's needed?

We need to standarize how we create clients, how to pass the destination to connect too, and how we close/cleanup the clients.

Proposed solution

Create a new base class called GrpcApiClient with at least some basic functionality, like:

class GrpcApiClient(Generic[T]):
        def __init__(self, target: str, create_stub: Callable[[GrpcChannel], T]):
            self._target = target
            self._channel = grpcaio.insecure_channel(target)
            self._stub: T = create_stub(self._grpc_channel)

    async def close(self, timeout: timedelta | None = None) -> None:
        await self._channel.close(grace=timeout.total_seconds() if timeout else None)

    async def __aenter__(self) -> "ReportingClient":
        return self

    async def __aexit__(
        self,
        _exc_type: Type[BaseException] | None,
        _exc_val: BaseException | None,
        _exc_tb: Any | None,
    ) -> bool | None:
        await self.close()
        return None

Use cases

No response

Alternatives and workarounds

Some clients take a grpc channel, and some a connection string. The advantage of taking a channel is users can chose which type of channel to use (insecure vs. secure for example), the disadvantage is we'll leak the dependency to grpc.

We could extend the connection string to chose if the channel should be secure or insecure, so we could have the best of both world. This would also enable to take connection strings directly from configuration files.

We might need a bit more research, maybe something like that already exists.

Additional context

• Introduce simple reporting client with support for single metric historical data frequenz-client-reporting-python#16 (comment)