Add metrics regarding buffer usage

[couloum]

Hi,

First of all, thanks guys for this great tool and for sharing it.

One feature of influxdb-relay is to buffer queries untill influxdb instance is up. This buffer is critical as if it gets full, we lose queries, and data. However, we have no visibility at all on the usage of this buffer.

It would be great if we could have prometheus metrics on the buffer usage.Ideally, we would need:

1. The total size of the buffer
2. The size usage of the buffer

And we would need this, per influxdb instance on which queries are relayed.

Thanks