feat: add AlwaysOffSampler (#72)

## Which problem is this PR solving?

- #70 added deterministic sampling but converted a sample rate of 0 to
1, but it may be nice to have the option keep the AlwaysOffSampler as an
option (we have this option in the node distro).

## Short description of the changes

- Allow sample rate of 0 and don't convert to default of 1
- Add AlwaysOffSampler to be used with sample rate of 0; drop
everything.
- Update tests

## How to verify that this has the expected result

Setting a sample rate of 0 is valid and should drop all telemetry

src/deterministic-sampler.ts

@@ -1,5 +1,6 @@
 import { Attributes, Context, Link, SpanKind } from '@opentelemetry/api';
 import {
+  AlwaysOffSampler,
   AlwaysOnSampler,
   Sampler,
   SamplingResult,
@@ -29,6 +30,10 @@ export class DeterministicSampler implements Sampler {
   constructor(sampleRate: number) {
     this._sampleRate = sampleRate;
     switch (sampleRate) {
+      // sample rate of 0 means send nothing
+      case 0:
+        this._sampler = new AlwaysOffSampler();
+        break;
       // sample rate of 1 is default, send everything
       case 1:
         this._sampler = new AlwaysOnSampler();

src/types.ts

@@ -72,7 +72,8 @@ export interface HoneycombOptions extends Partial<WebSDKConfiguration> {
   serviceName?: string;
 
   /** The sample rate used to determine whether a trace is exported.
-   * This must be a whole number greater than 1. Only 1 out of every `sampleRate` traces will be randomly selected to be sent.
+   * This must be a whole positive number. Only 1 out of every `sampleRate` traces will be randomly selected to be sent.
+   * Set to 0 to drop everything.
    * Defaults to 1 (send everything).
    */
   sampleRate?: number;

src/util.ts

@@ -71,10 +71,10 @@ export const getTracesApiKey = (options?: HoneycombOptions) => {
 
 export const getSampleRate = (options?: HoneycombOptions) => {
   if (
-    // sample rate must be a whole integer greater than 0
-    options?.sampleRate &&
-    options?.sampleRate > 0 &&
-    Number.isSafeInteger(options?.sampleRate)
+    // must be a whole positive integer
+    typeof options?.sampleRate === 'number' &&
+    Number.isSafeInteger(options?.sampleRate) &&
+    options?.sampleRate >= 0
   ) {
     return options?.sampleRate;
   }

test/deterministic-sampler.test.ts

@@ -65,22 +65,22 @@ describe('configureDeterministicSampler', () => {
     expect(result.attributes).toEqual({ SampleRate: 1 });
   });
 
-  test('sample rate of 0 configures inner AlwaysOnSampler', () => {
+  test('sample rate of 0 configures inner AlwaysOffSampler', () => {
     const options = {
       sampleRate: 0,
     };
     const sampler = configureDeterministicSampler(options);
     expect(sampler).toBeInstanceOf(DeterministicSampler);
-    expect(sampler.toString()).toBe('DeterministicSampler(AlwaysOnSampler)');
+    expect(sampler.toString()).toBe('DeterministicSampler(AlwaysOffSampler)');
 
     const result = getSamplingResult(sampler);
-    expect(result.decision).toBe(SamplingDecision.RECORD_AND_SAMPLED);
-    expect(result.attributes).toEqual({ SampleRate: 1 });
+    expect(result.decision).toBe(SamplingDecision.NOT_RECORD);
+    expect(result.attributes).toEqual({ SampleRate: 0 });
   });
 
   test('sample rate of -42 configures inner AlwaysOn Sampler', () => {
     const options = {
-      sampleRate: 0,
+      sampleRate: -42,
     };
     const sampler = configureDeterministicSampler(options);
     expect(sampler).toBeInstanceOf(DeterministicSampler);

test/util.test.ts

@@ -115,11 +115,11 @@ describe('sample rate', () => {
     expect(getSampleRate(options)).toBe(2);
   });
 
-  it('should use default sample rate if provided with 0', () => {
+  it('should use 0 sample rate if provided with 0', () => {
     const options = {
       sampleRate: 0,
     };
-    expect(getSampleRate(options)).toBe(1);
+    expect(getSampleRate(options)).toBe(0);
   });
 
   it('should use default sample rate if provided with negative', () => {