counter : increment/decrement metric counters and export them to an arrow record

Overview

• Field names and types must be specified at counter creation
• Keys must be of type integer, string or time.Time
• Metric counter is int64
• Use Increment() or Decrement() to modify, Get() to retrieve and Delete() to remove from counter, Reset() to clear all
• Export the metric results as an arrow record (ie. to write aggregated metrics to parquet)
• Uses concurrent swiss map as underlying cache, thread safe