fix(tracing): format _dd.p.ksr with decimal notation instead of scientific notation

packages/dd-trace/src/priority_sampler.js

@@ -37,13 +37,16 @@ const {
 const DEFAULT_KEY = 'service:,env:'
 
 /**
- * Formats a sampling rate as a string with up to 6 significant digits and no trailing zeros.
+ * Formats a sampling rate as a string with up to 6 decimal digits and no trailing zeros.
  *
  * @param {number} rate
- * @returns {string}
  */
 function formatKnuthRate (rate) {
-  return Number(rate.toPrecision(6)).toString()
+  const string = Number(rate).toFixed(6)
+  for (let i = string.length - 1; i > 0; i--) {
+    if (string[i] === '0') continue
+    return string.slice(0, i + (string[i] === '.' ? 0 : 1))
+  }
 }
 
 const defaultSampler = new Sampler(AUTO_KEEP)

packages/dd-trace/test/priority_sampler.spec.js

@@ -421,6 +421,45 @@ describe('PrioritySampler', () => {
       assert.strictEqual(typeof context._trace.tags[SAMPLING_KNUTH_RATE], 'string')
     })
 
+    it('should format _dd.p.ksr with decimal notation for very small rates', () => {
+      Sampler.withArgs(0.000001).returns({
+        isSampled: sinon.stub().returns(true),
+        rate: sinon.stub().returns(0.000001),
+      })
+      prioritySampler = new PrioritySampler('test', {
+        sampleRate: 0.000001,
+      })
+      prioritySampler.sample(span)
+
+      assert.strictEqual(context._trace.tags[SAMPLING_KNUTH_RATE], '0.000001')
+    })
+
+    it('should round _dd.p.ksr to zero when rate is below 6 decimal precision', () => {
+      Sampler.withArgs(0.0000001).returns({
+        isSampled: sinon.stub().returns(true),
+        rate: sinon.stub().returns(0.0000001),
+      })
+      prioritySampler = new PrioritySampler('test', {
+        sampleRate: 0.0000001,
+      })
+      prioritySampler.sample(span)
+
+      assert.strictEqual(context._trace.tags[SAMPLING_KNUTH_RATE], '0')
+    })
+
+    it('should round _dd.p.ksr up to 0.000001 when rate rounds up', () => {
+      Sampler.withArgs(0.00000051).returns({
+        isSampled: sinon.stub().returns(true),
+        rate: sinon.stub().returns(0.00000051),
+      })
+      prioritySampler = new PrioritySampler('test', {
+        sampleRate: 0.00000051,
+      })
+      prioritySampler.sample(span)
+
+      assert.strictEqual(context._trace.tags[SAMPLING_KNUTH_RATE], '0.000001')
+    })
+
     it('should not set _dd.p.ksr tag for manual sampling', () => {
       context._tags[MANUAL_KEEP] = undefined