Add byte rate metrics

[verdie-g]

For now it's only added to AsyncCommand. Closes #109.

[shannonklaus]

@verdie-g I have work in progress for adding a metrics feature to the client. There are two parts to it, one of them is the current info in ClusterStats and the second part is extended metrics including transaction count and latency buckets on a per node basis. I can add byte rate metrics to the extended metrics. Since what you are suggesting requires two atomic operations per transaction we don't want to have it enabled by default.

[verdie-g]

Since what you are suggesting requires two atomic operations per transaction we don't want to have it enabled by default.

Do you think we could go forward with that PR if I introduced a LowReadHighWriteAtomicCounter like https://gist.github.com/verdie-g/77b58548fa772fef42a710e4aea49576 as discussed in dotnet/runtime#92442?

[shannonklaus]

No, we would still like to wait until the metrics as I described are implemented and the bytes sent/received should only be enabled when metrics are enabled by the user.

[verdie-g]

Got it. Would you have an idea when these metrics could be available?

[shannonklaus]

I am currently planning on including it in the April release, which should be toward the end of April.