make AutoValue work in PubSub

This commit digs us out of the AutoValue dependency hell.

Previously
- We require Guava 20 for its Stats class.
- Guava source has a `@CanIgnoreReturnValue` annotation,
  this annotation is in errorprone package.
- Errorprone package required Java8 to run. So, I just took dependency
  on it without running to make the build pass in both 7 and 8.
- Errorprone shades class javax.lang.model.type.IntersectionType.
  This class is Java8-only, but because it wasn't run, it didn't cause a problem.
- AutoValue conditionally loads IntersectionType.
  It catches Exception and behaves properly if the class is not found;
  this is why it works on Java 7.
  However, because of the previous step, we have the class in our path.
  Because the class is 8-only, JVM7 loading it results in
  UnsupportedVersionError. This is an Error and not an Exception,
  and so AutoValue (probably wisely) ignore it and crashes.

The fix is simple.
Since #1480
already removed Stats, we can just go back to Guava 19.

@garrettjonesgoogle is full of wisdom!

Author: pongad

google-cloud-pubsub/pom.xml

@@ -53,16 +53,6 @@
       <artifactId>grpc-auth</artifactId>
       <version>${grpc.version}</version>
     </dependency>
-    <dependency>
-      <groupId>com.google.guava</groupId>
-      <artifactId>guava</artifactId>
-      <version>20.0</version>
-    </dependency>
-    <dependency>
-      <groupId>com.google.errorprone</groupId>
-      <artifactId>error_prone_core</artifactId>
-      <version>2.0.15</version>
-    </dependency>
     <dependency>
       <groupId>joda-time</groupId>
       <artifactId>joda-time</artifactId>
@@ -164,10 +154,7 @@
       </plugin>
       <plugin>
         <artifactId>maven-compiler-plugin</artifactId>
-        <!-- Downgrading to 3.1 because of https://issues.apache.org/jira/browse/MCOMPILER-236 -->
-        <!-- Upgrade to 3.5.1 which fixes the problem when available -->
-        <!-- <version>3.5.1</version> -->
-        <version>3.1</version>
+        <version>3.5.1</version>
         <configuration>
           <source>1.7</source>
           <target>1.7</target>

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/PublisherStats.java

@@ -16,44 +16,24 @@
 
 package com.google.cloud.pubsub;
 
-import javax.annotation.concurrent.Immutable;
+import com.google.auto.value.AutoValue;
 
 /**
  * A snapshot of the publisher statistics at the time they were requested from the {@link
  * Publisher}.
  */
 //TODO: Finish implementation.
-@Immutable
-public class PublisherStats {
-  private final long sentMessages;
-  private final long ackedMessages;
-  private final long failedMessages;
-  private final long pendingMessages;
-
-  PublisherStats(long sentMessages, long ackedMessages, long failedMessages, long pendingMessages) {
-    this.sentMessages = sentMessages;
-    this.ackedMessages = ackedMessages;
-    this.failedMessages = failedMessages;
-    this.pendingMessages = pendingMessages;
-  }
-
+@AutoValue
+public abstract class PublisherStats {
   /** Number of successfully published messages. */
-  public long getAckedMessages() {
-    return ackedMessages;
-  }
+  public abstract long getAckedMessages();
 
   /** Number of messages that failed to publish. */
-  public long getFailedMessages() {
-    return failedMessages;
-  }
+  public abstract long getFailedMessages();
 
   /** Number of messages pending to publish, includes message in-flight. */
-  public long getPendingMessages() {
-    return pendingMessages;
-  }
+  public abstract long getPendingMessages();
 
   /** Total messages sent, equal to pending + acked + failed messages. */
-  public long getSentMessages() {
-    return sentMessages;
-  }
+  public abstract long getSentMessages();
 }

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/SubscriberStats.java

@@ -16,67 +16,39 @@
 
 package com.google.cloud.pubsub;
 
-import javax.annotation.concurrent.Immutable;
+import com.google.auto.value.AutoValue;
 
 /**
  * A snapshot of the subscriber statistics at the time they were requested from the {@link
  * Subscriber}.
  */
 //TODO: Finish implementation.
-@Immutable
-public class SubscriberStats {
-  @Immutable
-  public static class Stats {}
-
-  private final long totalReceivedMessages;
-  private final long totalAckedMessages;
-  private final Stats endToEndLatency;
-  private final Stats ackLatency;
-  private final long numberOfAutoExtendedAckDeadlines;
-
-  SubscriberStats() {
-    this.totalReceivedMessages = 0;
-    this.totalAckedMessages = 0;
-    this.numberOfAutoExtendedAckDeadlines = 0;
-    this.endToEndLatency = null;
-    this.ackLatency = null;
-  }
+@AutoValue
+public abstract class SubscriberStats {
+  @AutoValue
+  public abstract static class Stats {}
 
   /** Number of successfully published messages. */
-  public long getReceivedMessages() {
-    return totalReceivedMessages;
-  }
+  public abstract long getReceivedMessages();
 
   /** Number of successfully published messages. */
-  public long getAckedMessages() {
-    return totalAckedMessages;
-  }
+  public abstract long getAckedMessages();
 
   /** Number of received messages. */
-  public long getTotalReceivedMessages() {
-    return totalReceivedMessages;
-  }
+  public abstract long getTotalReceivedMessages();
 
   /** Number messages acked. */
-  public long getTotalAckedMessages() {
-    return totalAckedMessages;
-  }
+  public abstract long getTotalAckedMessages();
 
   /** End to end latency. */
-  public Stats getEndToEndLatency() {
-    return endToEndLatency;
-  }
+  public abstract Stats getEndToEndLatency();
 
   /**
    * Acknowledgement latency; time in between the message has been received and then acknowledged or
    * rejected.
    */
-  public Stats getAckLatency() {
-    return ackLatency;
-  }
+  public abstract Stats getAckLatency();
 
   /** Number of messages for which we have auto extended its acknowledgement deadline. */
-  public long getNumberOfAutoExtendedAckDeadlines() {
-    return numberOfAutoExtendedAckDeadlines;
-  }
+  public abstract long getNumberOfAutoExtendedAckDeadlines();
 }