fix: Prevent filtered traces from biasing the sample rate (#1018)

The `URLFilter` and `MethodsFilter` implementations are side-effect
free, and are safe to run in any order. However, `sampler.shouldTrace`
is not, a result of `true` from it has the side effect of altering the
trace window.

This commit fixes this by calling the side-effecting sampler method last.

src/tracing-policy.ts

@@ -117,9 +117,9 @@ export class TracePolicy {
    */
   shouldTrace(options: {timestamp: number, url: string, method: string}):
       boolean {
-    return this.sampler.shouldTrace(options.timestamp) &&
-        this.urlFilter.shouldTrace(options.url) &&
-        this.methodsFilter.shouldTrace(options.method);
+    return this.urlFilter.shouldTrace(options.url) &&
+        this.methodsFilter.shouldTrace(options.method) &&
+        this.sampler.shouldTrace(options.timestamp);
   }
 
   static always(): TracePolicy {