feat: add range based exponential buckets in histogram

Cargo.toml

@@ -24,6 +24,7 @@ parking_lot = "0.12"
 prometheus-client-derive-encode = { version = "0.4.1", path = "derive-encode" }
 prost = { version = "0.12.0", optional = true }
 prost-types = { version = "0.12.0", optional = true }
+log = "0.4.22"
 
 [dev-dependencies]
 async-std = { version = "1", features = ["attributes"] }

src/metrics/histogram.rs

@@ -5,6 +5,7 @@
 use crate::encoding::{EncodeMetric, MetricEncoder, NoLabelSet};
 
 use super::{MetricType, TypedMetric};
+use log::warn;
 use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard};
 use std::iter::{self, once};
 use std::sync::Arc;
@@ -122,6 +123,33 @@ pub fn exponential_buckets(start: f64, factor: f64, length: u16) -> impl Iterato
         .take(length.into())
 }
 
+/// Exponential bucket distribution within a range
+///
+/// Creates `length` buckets, where the lowest bucket is `min` and the highest bucket is `max`.
+///
+/// The function defaults to `length` = 1 if `length` is 0 or negative,
+/// and defaults to `min` = 1.0 if `min` is 0 or negative.
+pub fn exponential_buckets_range(min: f64, max: f64, length: u16) -> impl Iterator<Item = f64> {
+    let mut len_observed = length;
+    let mut min_bucket = min;
+    if length < 1 {
+        warn!("exponential_buckets_range length needs a positive length, defaulting to 1");
+        len_observed = 1;
+    }
+    if min <= 0.0 {
+        warn!("exponential_buckets_range min needs to be greater than 0, defaulting to 1.0");
+        min_bucket = 1.0;
+    }
+
+    // We know max/min and highest bucket. Solve for growth_factor.
+    let growth_factor = (max / min_bucket).powf(1.0 / (len_observed as f64 - 1.0));
+
+    iter::repeat(())
+        .enumerate()
+        .map(move |(i, _)| min_bucket * growth_factor.powf(i as f64))
+        .take(len_observed.into())
+}
+
 /// Linear bucket distribution.
 pub fn linear_buckets(start: f64, width: f64, length: u16) -> impl Iterator<Item = f64> {
     iter::repeat(())
@@ -166,4 +194,12 @@ mod tests {
             linear_buckets(0.0, 1.0, 10).collect::<Vec<_>>()
         );
     }
+
+    #[test]
+    fn exponential_range() {
+        assert_eq!(
+            vec![1.0, 2.0, 4.0, 8.0, 16.0, 32.0],
+            exponential_buckets_range(1.0, 32.0, 6).collect::<Vec<_>>()
+        );
+    }
 }