Introduce a metric to track the number of bytes we're sending to Kusto

[jessejlt]

No description provided.

[mkeesey]

I don't think this will quite do what we want. This gauge will get its value overwritten every time it uploads a segment to whatever size a given segment is, so we'll get a lot of different values for this gauge that don't really correspond to any consistent view of how much we're uploading.

I think if we want to keep track of the bytes being uploaded, this needs to be a Counter that we always append to. This way we're keeping track of the total as time goes on, and the multiple goroutines that write the metric won't step on each other and overwrite each other's value.