function _merge(a, b) {
  for (const name in b) {
    if (a[name]) {
      a[name].merge(b[name])
    } else {
      a[name] = b[name]
    }
  }
}
Combine two sets of metric data. Intended to be used as described above,
either when folding a transaction's metrics into the agent's metrics for
later harvest, or one harvest cycle's metrics into the next when a
delivery attempt to the collector fails. Among the more performance-
critical pieces of code in the agent, so some performance tuning would
probably be a good idea.

@param {Metrics} other
 The collection to be folded into this one.
@param {boolean} adjustStartTime
 If the start time for the timeslice should be adjusted.