MetricData

MetricData is an aggregated snapshot of one instrument at one collect cycle — its accumulated points, keyed by label set. You rarely build it directly: it is what MetricReader.collectAllMetrics() returns, one value per registered instrument, for a test to assert on or an export pipeline to serialize. Each collection builds fresh points, though a histogram point exposes its boundaries and bucketCounts arrays as-is, so read them and don't write to them.

One thing it does not carry is the instrument's name, description, or unit — a snapshot is only points and their label sets. So collectAllMetrics() hands back a Seq[MetricData] with no way to tell which instrument each entry came from, which is why an export pipeline pairs them with names of its own before serializing. In a test, collect from a single instrument with its own collect() when you need to know the source.

Its three variants line up with the instrument that produced them:

• SumData — from a Counter or an UpDownCounter
• HistogramData — from a Histogram
• GaugeData — from a Gauge

So a match on MetricData tells you both the shape of the numbers and the kind of instrument they came from.

sealed trait MetricData

object MetricData {
  final case class SumData(points: List[SumDataPoint])             extends MetricData
  final case class HistogramData(points: List[HistogramDataPoint]) extends MetricData
  final case class GaugeData(points: List[GaugeDataPoint])         extends MetricData
}

final case class SumDataPoint(
  attributes:     Attributes,
  startTimeNanos: Long,
  timeNanos:      Long,
  value:          Long          // cumulative sum
)

final case class HistogramDataPoint(
  attributes:     Attributes,
  startTimeNanos: Long,
  timeNanos:      Long,
  count:          Long,         // total number of observations
  sum:            Double,       // sum of all observations
  min:            Double,       // smallest observation
  max:            Double,       // largest observation
  bucketCounts:   Array[Long],  // count per bucket (length == boundaries.length + 1)
  boundaries:     Array[Double] // the bucket upper boundaries
)

final case class GaugeDataPoint(
  attributes: Attributes,
  timeNanos:  Long,
  value:      Double            // most recently recorded value
)