Histogram bucket helpers

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

lib/prometheus/client.rb

@@ -14,5 +14,13 @@ def self.registry
     def self.config
       @config ||= Config.new
     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
   end
 end

spec/prometheus/client_spec.rb

@@ -12,4 +12,20 @@
       expect(Prometheus::Client.registry).to eql(Prometheus::Client.registry)
     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
 end