feat: allow users to specify a propagation mechanism

Author: kjin

src/config.ts

@@ -213,6 +213,13 @@ export interface Config {
    */
   tracePolicy?: TracePolicy;
 
+  /**
+   * If specified, the Trace Agent will use this context header propagation
+   * implementation instead of @opencensus/propagation-stackdriver, the default
+   * trace context header format.
+   */
+  propagation?: OpenCensusPropagation;
+
   /**
    * Buffer the captured traces for `flushDelaySeconds` seconds before
    * publishing to the Stackdriver Trace API, unless the buffer fills up first.

src/index.ts

@@ -146,7 +146,10 @@ function initConfig(userConfig: Forceable<Config>): Forceable<TopLevelConfig> {
       contextHeaderBehavior: mergedConfig.contextHeaderBehavior as
           TraceContextHeaderBehavior
     },
-    overrides: {tracePolicy: mergedConfig.tracePolicy}
+    overrides: {
+      tracePolicy: mergedConfig.tracePolicy,
+      propagation: mergedConfig.propagation
+    }
   };
 }
 

src/tracing.ts

@@ -18,7 +18,7 @@ import {v1 as stackdriverPropagation} from '@opencensus/propagation-stackdriver'
 import * as path from 'path';
 
 import {cls, TraceCLSConfig} from './cls';
-import {TracePolicy} from './config';
+import {OpenCensusPropagation, TracePolicy} from './config';
 import {LEVELS, Logger} from './logger';
 import {StackdriverTracer} from './trace-api';
 import {pluginLoader, PluginLoaderConfig} from './trace-plugin-loader';
@@ -31,7 +31,8 @@ export type TopLevelConfig = {
   writerConfig: TraceWriterConfig;
   pluginLoaderConfig: PluginLoaderConfig;
   tracePolicyConfig: TracePolicyConfig;
-  overrides: {tracePolicy?: TracePolicy;};
+  overrides:
+      {tracePolicy?: TracePolicy; propagation?: OpenCensusPropagation;};
 }|{
   enabled: false;
 };
@@ -121,7 +122,8 @@ export class Tracing implements Component {
 
     const tracePolicy = this.config.overrides.tracePolicy ||
         new BuiltinTracePolicy(this.config.tracePolicyConfig);
-    const propagation = stackdriverPropagation;
+    const propagation =
+        this.config.overrides.propagation || stackdriverPropagation;
 
     const tracerComponents = {logger: this.logger, tracePolicy, propagation};