Histogram bucket helpers

Most client libraries provide helpers for generating linear/exponential
histogram buckets. This provides the same interface as the golang
client.

Signed-off-by: Lawrence Jones <lawrjone@gmail.com>

Author: lawrencejones

README.md

@@ -153,6 +153,11 @@ histogram.get(labels: { service: 'users' })
 # => { 0.005 => 3, 0.01 => 15, 0.025 => 18, ..., 2.5 => 42, 5 => 42, 10 = >42 }
 ```
 
+Histograms provide default buckets of `[0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10]`
+
+You can specify your own buckets, either explicitly, or using the `Histogram.linear_buckets`
+or `Histogram.exponential_buckets` methods to define regularly spaced buckets.
+
 ### Summary
 
 Summary, similar to histograms, is an accumulator for samples. It captures

lib/prometheus/client/histogram.rb

@@ -33,6 +33,14 @@ def initialize(name,
               store_settings: store_settings)
       end
 
+      def self.linear_buckets(start:, width:, count:)
+        count.times.map { |idx| start.to_f + idx * width }
+      end
+
+      def self.exponential_buckets(start:, factor: 2, count:)
+        count.times.map { |idx| start.to_f * factor ** idx }
+      end
+
       def with_labels(labels)
         self.class.new(name,
                        docstring: docstring,

spec/prometheus/client/histogram_spec.rb

@@ -37,6 +37,20 @@
     end
   end
 
+  describe ".linear_buckets" do
+    it "generates buckets" do
+      expect(described_class.linear_buckets(start: 1, width: 2, count: 5)).
+        to eql([1.0, 3.0, 5.0, 7.0, 9.0])
+    end
+  end
+
+  describe ".exponential_buckets" do
+    it "generates buckets" do
+      expect(described_class.exponential_buckets(start: 1, factor: 2, count: 5)).
+        to eql([1.0, 2.0, 4.0, 8.0, 16.0])
+    end
+  end
+
   describe '#observe' do
     it 'records the given value' do
       expect do