Struct HistogramValue 

pub struct HistogramValue {
    pub reservoir: Arc<Histogram<u64>>,
    pub count: u64,
    pub cumulative_count: u64,
    pub sum: f64,
    pub created: Option<Instant>,
    pub bucket_exemplars: Vec<Option<Exemplar>>,
}

OpenMetrics §5.3.1 histogram point. Internally carries the HDR reservoir as the source of truth — OpenMetrics-shaped cumulative buckets are derived on demand at exposition time.

sum / count are derivable from the reservoir but cached for O(1) access. created is the series start time (component start) per spec.

Per-bucket exemplars are NOT stored on the reservoir — they live in HistogramValue::bucket_exemplars keyed by bucket index of the consumer’s eventual bucket layout. (Combine semantics: most-recent-wins by MetricPoint.timestamp, per SRD-42 §“Exemplars → Combine semantics”.)

Fields

reservoir: Arc<Histogram<u64>>

HDR reservoir — the lossless source of truth for combining across cascade folds and ephemeral merges.

count: u64

Cached observation count. Equal to reservoir.len() — the per-window count in a delta snapshot.

cumulative_count: u64

The cumulative total observation count at this point’s timestamp (monotonic, Prometheus/VM-schematic), carried alongside the per-window count exactly as CounterValue carries cumulative alongside total. The queryapi exposes this so MetricsQL rate()/increase/*_over_time over a histogram’s count are PromQL-correct. Time-coalesce keeps the latest; cross-component aggregate sums it. (The percentile reservoir stays windowed — see the cumulative-counter note.)

sum: f64

Cached observation sum (nanoseconds for latency timers).

created: Option<Instant>

Series start time per spec §5.3; optional.

bucket_exemplars: Vec<Option<Exemplar>>

Sampled exemplars, one per bucket of an eventual exposition layout. Sparse: an empty slot means “no exemplar for that bucket”. See SRD-42 §“Exemplars”.

Implementations

impl HistogramValue

pub fn from_hdr(reservoir: HdrHistogram<u64>) -> Self

pub fn with_cumulative_count(self, cumulative_count: u64) -> Self

Override the cumulative observation count (the cadence capture path sets the accumulated absolute total here).

pub fn with_created(self, t: Instant) -> Self

pub fn with_bucket_exemplars(self, exemplars: Vec<Option<Exemplar>>) -> Self

pub fn project_buckets(&self, bounds: &[u64]) -> Vec<Bucket>

Project to OpenMetrics-shaped cumulative buckets at the given upper bounds. Per spec §5.3, the final bucket MUST have upper_bound = +Inf; this helper appends it automatically if not present.

bounds should be sorted ascending. Returns (upper_bound, cumulative_count) pairs.

Trait Implementations

impl Clone for HistogramValue

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for HistogramValue

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.