Provide an SPI that an agent distribution or intiializer can use to c… (#561)

agent-bootstrap/agent-bootstrap.gradle

@@ -12,6 +12,7 @@ minimumInstructionCoverage = 0.0
 dependencies {
   compile deps.opentelemetryApi
   compile deps.opentelemetryApiAutoAnnotations
+  compileOnly deps.opentelemetrySdk
   compile project(':utils:thread-utils')
   compile deps.slf4j
   compile group: 'org.slf4j', name: 'slf4j-simple', version: versions.slf4j

agent-bootstrap/src/main/java/io/opentelemetry/auto/bootstrap/spi/TracerCustomizer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.opentelemetry.auto.bootstrap.spi;
+
+import io.opentelemetry.context.propagation.ContextPropagators;
+import io.opentelemetry.sdk.trace.TracerSdkProvider;
+import io.opentelemetry.sdk.trace.config.TraceConfig;
+
+/**
+ * A service provider to allow programmatic customization of the tracing configuration. It will be
+ * executed after the SDK has initialized a {@link TracerSdkProvider}. This means that not only can
+ * the provided {@link TracerSdkProvider} be configured e.g. by calling {@link
+ * TracerSdkProvider#updateActiveTraceConfig(TraceConfig)}, but static methods on {@link
+ * io.opentelemetry.OpenTelemetry}, e.g., {@link
+ * io.opentelemetry.OpenTelemetry#setPropagators(ContextPropagators)} can be used as well.
+ *
+ * <p>An implementation of {@link TracerCustomizer} can either be provided as part of an initializer
+ * JAR, using the {@code ota.initializer.jar} property or can be included in the same JAR as the
+ * agent in a redistribution.
+ */
+public interface TracerCustomizer {
+
+  /** Callback executed after the initial {@link TracerSdkProvider} has been initialized. */
+  void configure(TracerSdkProvider tracerSdkProvider);
+}

agent-tooling/src/main/java/io/opentelemetry/auto/tooling/TracerInstaller.java

@@ -15,12 +15,14 @@
  */
 package io.opentelemetry.auto.tooling;
 
+import io.opentelemetry.auto.bootstrap.spi.TracerCustomizer;
 import io.opentelemetry.auto.config.Config;
 import io.opentelemetry.sdk.OpenTelemetrySdk;
 import io.opentelemetry.sdk.contrib.auto.config.MetricExporterFactory;
 import io.opentelemetry.sdk.contrib.auto.config.SpanExporterFactory;
 import io.opentelemetry.sdk.metrics.export.IntervalMetricReader;
 import io.opentelemetry.sdk.metrics.export.MetricExporter;
+import io.opentelemetry.sdk.trace.TracerSdkProvider;
 import io.opentelemetry.sdk.trace.config.TraceConfig;
 import io.opentelemetry.sdk.trace.export.BatchSpanProcessor;
 import io.opentelemetry.sdk.trace.export.SpanExporter;
@@ -152,16 +154,18 @@ public class TracerInstaller {
   }
 
   private static void configure() {
+    // Execute any user-provided (usually vendor-provided) configuration logic.
+    final ServiceLoader<TracerCustomizer> serviceLoader =
+        ServiceLoader.load(TracerCustomizer.class, TracerInstaller.class.getClassLoader());
+    final TracerSdkProvider tracerSdkProvider = OpenTelemetrySdk.getTracerProvider();
+    for (TracerCustomizer customizer : serviceLoader) {
+      customizer.configure(tracerSdkProvider);
+    }
+
     /* Update trace config from env vars or sys props */
-    final TraceConfig activeTraceConfig =
-        OpenTelemetrySdk.getTracerProvider().getActiveTraceConfig();
-    OpenTelemetrySdk.getTracerProvider()
-        .updateActiveTraceConfig(
-            activeTraceConfig
-                .toBuilder()
-                .readEnvironmentVariables()
-                .readSystemProperties()
-                .build());
+    final TraceConfig activeTraceConfig = tracerSdkProvider.getActiveTraceConfig();
+    tracerSdkProvider.updateActiveTraceConfig(
+        activeTraceConfig.toBuilder().readEnvironmentVariables().readSystemProperties().build());
   }
 
   @SuppressWarnings("unused")