fix: Use monotonic clock to compute durations (#2441)

A monotonic clock is monotonically increasing and not subject to system
clock adjustments or system clock skew. The difference between any two
chronologically recorded time values returned from the Performance.now()
method MUST never be negative if the two time values have the same time
origin.

The same guarantee above does not exist for the difference between two
calls to `new Date().getTime()` as used by `timestampWithMs()`.

Resources:

https://stackoverflow.com/questions/7272395/monotonically-increasing-time-in-javascript
https://caniuse.com/#search=performance.now
https://www.w3.org/TR/hr-time/#sec-monotonic-clock

Co-authored-by: Kamil Ogórek <kamil.ogorek@gmail.com>

Author: rhcarvalho

packages/apm/src/span.ts

@@ -2,7 +2,26 @@
 
 import { getCurrentHub, Hub } from '@sentry/hub';
 import { Span as SpanInterface, SpanContext, SpanStatus } from '@sentry/types';
-import { dropUndefinedKeys, isInstanceOf, logger, timestampWithMs, uuid4 } from '@sentry/utils';
+import {
+  dropUndefinedKeys,
+  dynamicRequire,
+  getGlobalObject,
+  isInstanceOf,
+  isNodeEnv,
+  logger,
+  timestampWithMs,
+  uuid4,
+} from '@sentry/utils';
+
+const global = getGlobalObject<Window>();
+
+const performanceNow = (() => {
+  if (isNodeEnv()) {
+    const { performance } = dynamicRequire(module, 'perf_hooks');
+    return performance.now;
+  }
+  return global.performance.now.bind(global.performance);
+})();
 
 // TODO: Should this be exported?
 export const TRACEPARENT_REGEXP = new RegExp(
@@ -81,6 +100,17 @@ export class Span implements SpanInterface, SpanContext {
    */
   public readonly startTimestamp: number = timestampWithMs();
 
+  /**
+   * Internal start time tracked with a monotonic clock.
+   *
+   * Works with mostly any browser version released since 2012.
+   * https://caniuse.com/#search=performance.now
+   *
+   * Works with Node.js v8.5.0 or higher.
+   * https://nodejs.org/api/perf_hooks.html#perf_hooks_performance_now
+   */
+  private readonly _startTimestampMonotonic: number = performanceNow();
+
   /**
    * Finish timestamp of the span.
    */
@@ -261,7 +291,8 @@ export class Span implements SpanInterface, SpanContext {
       return undefined;
     }
 
-    this.timestamp = timestampWithMs();
+    const durationSeconds = (performanceNow() - this._startTimestampMonotonic) / 1000;
+    this.timestamp = this.startTimestamp + durationSeconds;
 
     if (this.spanRecorder === undefined) {
       return undefined;