txmetrics: remove timeseries.instance field

[axw]

The timeseries.instance field is not used anywhere. Eventually we should move these metrics to using TSDB, which generates a timeseries ID from the dimensions.

[simitt]

Wasn't the original intention to use this e.g. for telemetry to get a better understanding of usage and potential issues with the metrics aggregation?

[axw]

@simitt I don't recall tbqh. If it was, we could also do that with TSDB using the _tsid field. We could alternatively compare transaction event count to transaction metric count for a rough approximation.