Make datadog distributed tracing header take priority over tracecontext (#3414)

Author: Stephen Belanger

packages/dd-trace/src/config.js

@@ -278,7 +278,7 @@ class Config {
       process.env.DD_TRACE_EXPERIMENTAL_B3_ENABLED,
       false
     )
-    const defaultPropagationStyle = ['tracecontext', 'datadog']
+    const defaultPropagationStyle = ['datadog', 'tracecontext']
     if (isTrue(DD_TRACE_B3_ENABLED)) {
       defaultPropagationStyle.push('b3')
       defaultPropagationStyle.push('b3 single header')

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

@@ -95,8 +95,8 @@ describe('Config', () => {
     expect(config).to.have.property('spanComputePeerService', false)
     expect(config).to.have.property('traceRemoveIntegrationServiceNamesEnabled', false)
     expect(config).to.have.deep.property('serviceMapping', {})
-    expect(config).to.have.nested.deep.property('tracePropagationStyle.inject', ['tracecontext', 'datadog'])
-    expect(config).to.have.nested.deep.property('tracePropagationStyle.extract', ['tracecontext', 'datadog'])
+    expect(config).to.have.nested.deep.property('tracePropagationStyle.inject', ['datadog', 'tracecontext'])
+    expect(config).to.have.nested.deep.property('tracePropagationStyle.extract', ['datadog', 'tracecontext'])
     expect(config).to.have.nested.property('experimental.runtimeId', false)
     expect(config).to.have.nested.property('experimental.exporter', undefined)
     expect(config).to.have.nested.property('experimental.enableGetRumData', false)