Fix gRPC Context shading (#297)

instrumentation/grpc-1.5/grpc-1.5.gradle

@@ -53,4 +53,11 @@ dependencies {
   latestDepTestCompile group: 'io.grpc', name: 'grpc-netty', version: '+'
   latestDepTestCompile group: 'io.grpc', name: 'grpc-protobuf', version: '+'
   latestDepTestCompile group: 'io.grpc', name: 'grpc-stub', version: '+'
+
+  // this instrumentation needs to be able to be able to reference the OpenTelemetry API's gRPC Context
+  // that is shaded in the bootstrap class loader (for sending telemetry to the agent),
+  // separately from the gRPC Context that is brought by gRPC
+  compileOnly project(path: ':opentelemetry-api-shaded-for-instrumenting', configuration: 'shadow')
+
+  testCompile project(path: ':opentelemetry-api-shaded-for-instrumenting', configuration: 'shadow')
 }

instrumentation/grpc-1.5/src/main/java/io/opentelemetry/auto/instrumentation/grpc/client/TracingClientInterceptor.java

@@ -86,6 +86,11 @@ static final class TracingClientCall<ReqT, RespT>
 
     @Override
     public void start(final Listener<RespT> responseListener, final Metadata headers) {
+      // this reference to io.grpc.Context will be shaded during the build
+      // see instrumentation.gradle: "relocate OpenTelemetry API dependency usage"
+      // (luckily the grpc instrumentation doesn't need to reference unshaded grpc Context, so we
+      // don't need to worry about distinguishing them like in the opentelemetry-api
+      // instrumentation)
       final Context context = withSpan(span, Context.current());
       OpenTelemetry.getPropagators().getHttpTextFormat().inject(context, headers, SETTER);
       try (final Scope scope = currentContextWith(span)) {

instrumentation/instrumentation.gradle

@@ -94,9 +94,10 @@ shadowJar {
   relocate "io.opentelemetry.metrics", "io.opentelemetry.auto.shaded.io.opentelemetry.metrics"
   relocate "io.opentelemetry.trace", "io.opentelemetry.auto.shaded.io.opentelemetry.trace"
 
-  // relocate OpenTelemetry API dependency
-  relocate "io.grpc", "io.opentelemetry.auto.shaded.io.grpc"
+  // relocate OpenTelemetry API dependency usage
+  relocate "io.grpc.Context", "io.opentelemetry.auto.shaded.io.grpc.Context"
 
   // this is for instrumentation on opentelemetry-api itself
   relocate "unshaded.io.opentelemetry", "io.opentelemetry"
+  relocate "unshaded.io.grpc", "io.grpc"
 }