Add time averaged stats for future alarm list work.

This is a trivial C89-ification of the []2 implementation of the same idea.
Indeed the implementation files and tests have been branched from the [] versions.
	Change on 2014/12/15 by ctiller <ctiller@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82168822
diff --git a/build.json b/build.json
index df12434..0b2c684 100644
--- a/build.json
+++ b/build.json
@@ -127,6 +127,7 @@
         "src/core/iomgr/tcp_client_posix.c",
         "src/core/iomgr/tcp_posix.c",
         "src/core/iomgr/tcp_server_posix.c",
+        "src/core/iomgr/time_averaged_stats.c",
         "src/core/security/auth.c",
         "src/core/security/base64.c",
         "src/core/security/credentials.c",
@@ -222,6 +223,7 @@
         "src/core/iomgr/tcp_client.h",
         "src/core/iomgr/tcp_posix.h",
         "src/core/iomgr/tcp_server.h",
+        "src/core/iomgr/time_averaged_stats.h",
         "src/core/security/auth.h",
         "src/core/security/base64.h",
         "src/core/security/credentials.h",
@@ -582,6 +584,18 @@
       ]
     },
     {
+      "name": "time_averaged_stats_test",
+      "build": "test",
+      "src": [
+        "test/core/iomgr/time_averaged_stats_test.c"
+      ],
+      "deps": [
+        "grpc_test_util",
+        "grpc",
+        "gpr"
+      ]
+    },
+    {
       "name": "chttp2_stream_encoder_test",
       "build": "test",
       "src": [