core: Bump up instrumentation_api to 0.4.3, use instrumentation-defined STATS_CONTEXT_KEY. (#3070)

diff --git a/build.gradle b/build.gradle
index 7821587..81458df 100644
--- a/build.gradle
+++ b/build.gradle
@@ -184,7 +184,7 @@
                 google_auth_credentials: 'com.google.auth:google-auth-library-credentials:0.4.0',
                 okhttp: 'com.squareup.okhttp:okhttp:2.5.0',
                 okio: 'com.squareup.okio:okio:1.6.0',
-                instrumentation_api: 'com.google.instrumentation:instrumentation-api:0.4.2',
+                instrumentation_api: 'com.google.instrumentation:instrumentation-api:0.4.3',
                 protobuf: "com.google.protobuf:protobuf-java:${protobufVersion}",
                 protobuf_lite: "com.google.protobuf:protobuf-lite:3.0.1",
                 protoc_lite: "com.google.protobuf:protoc-gen-javalite:3.0.0",