Add trace key length as metric

[MikeGoldsmith]

The trace key builder was updated to have a maximum number of unique values. This issue tracks exposing the number of values used to build a trace key as a metric, likely a histogram.

---

          OK; the function here doesn't have access to a logger, so I had it return the key size it's using and log if it is the max value.

It occurs to me that we could now make a metric out of this, but I don't have time to do that. Please feel free if you think that's a good idea.

Originally posted by @kentquirk in #1364 (comment)