When spans are kept due to Dynsampler Keyspace exhaustion, note it on the spans

[mterhar]

Is your feature request related to a problem? Please describe.
Currently, when a trace is kept due to dynsampler keyspace exhaustion, it doesn't show in honeycomb.

Describe the solution you'd like
Include the rule that matched and the fact that keyspace was exhausted.

Describe alternatives you've considered
Make a "everything else" bucket for spans that match exhausted Keyspaces so they still get sampled a bit, even if it's not perfect.

Additional context

[codeboten]

Would adding a metric around keyspace exhaustion errors be a helpful thing to have here?