HADOOP-17271. S3A connector to support IOStatistics. (apache#2580)

S3A connector to support the IOStatistics API of HADOOP-16830,

This is a major rework of the S3A Statistics collection to

* Embrace the IOStatistics APIs
* Move from direct references of S3AInstrumention statistics
  collectors to interface/implementation classes in new packages.
* Ubiquitous support of IOStatistics, including:
  S3AFileSystem, input and output streams, RemoteIterator instances
  provided in list calls.
* Adoption of new statistic names from hadoop-common

Regarding statistic collection, as well as all existing
statistics, the connector now records min/max/mean durations
of HTTP GET and HEAD requests, and those of LIST operations.

Contributed by Steve Loughran.

Change-Id: I182d34b6ac39e017a8b4a221dad8e930882b39cf

Author: steveloughran

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/Constants.java

@@ -735,6 +735,13 @@ private Constants() {
   public static final String STREAM_READ_GAUGE_INPUT_POLICY =
       "stream_read_gauge_input_policy";
 
+  /**
+   * Gauge name for the input policy : {@value}.
+   * This references an enum currently exclusive to the S3A stream.
+   */
+  public static final String STREAM_READ_GAUGE_INPUT_POLICY =
+      "stream_read_gauge_input_policy";
+
   @InterfaceAudience.Private
   @InterfaceStability.Unstable
   public static final String S3_CLIENT_FACTORY_IMPL =