Record number of spans in a trace

[cartermp]

This came about in a meeting with the .NET team about trace depth.

We know that a lot of customers mostly just use automatic instrumentation and maybe add some metadata to the current span here or there. We also know that some customers go all-in on tracing and instrument very deeply.

It would be useful to look at a distribution of num_spans_per_trace and then slice & dice this based on another dimension, like which language they're instrumenting with for OTel, or something else.