core: change Metadata internals to avoid garbage creation

Before:
Benchmark                                                                   (headerCount)    Mode     Cnt        Score    Error  Units
InboundHeadersBenchmark.defaultHeaders_clientHandler                                  N/A    avgt      10      240.879 ±  4.903  ns/op
InboundHeadersBenchmark.defaultHeaders_serverHandler                                  N/A    avgt      10      882.354 ± 16.177  ns/op
InboundHeadersBenchmark.grpcHeaders_clientHandler                                     N/A    avgt      10      208.068 ±  5.380  ns/op
InboundHeadersBenchmark.grpcHeaders_serverHandler                                     N/A    avgt      10      477.604 ±  8.200  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                           1  sample  234233      125.232 ± 11.903  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                           5  sample  344367      264.343 ± 18.318  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                          10  sample  392273      439.640 ±  2.589  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                          20  sample  221506      855.115 ± 38.899  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                           1  sample  253676      111.941 ±  2.742  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                           5  sample  368499      248.255 ±  2.601  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                          10  sample  390015      439.651 ± 11.040  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                          20  sample  221807      840.435 ± 21.667  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                            1  sample  230139      432.866 ± 25.503  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                            5  sample  226901      765.095 ± 19.969  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                           10  sample  260495     1268.239 ± 21.850  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                           20  sample  311526     2059.973 ± 23.503  ns/op

After:

Benchmark                                                                   (headerCount)    Mode     Cnt        Score    Error  Units
InboundHeadersBenchmark.defaultHeaders_clientHandler                                  N/A    avgt      10      104.317 ±  1.973  ns/op
InboundHeadersBenchmark.defaultHeaders_serverHandler                                  N/A    avgt      10      395.666 ± 11.056  ns/op
InboundHeadersBenchmark.grpcHeaders_clientHandler                                     N/A    avgt      10       64.147 ±  4.076  ns/op
InboundHeadersBenchmark.grpcHeaders_serverHandler                                     N/A    avgt      10      228.299 ±  2.874  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                           1  sample  252451      102.718 ±  2.714  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                           5  sample  239976      225.812 ± 38.824  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                          10  sample  258119      364.475 ± 57.217  ns/op
OutboundHeadersBenchmark.convertClientHeaders                                          20  sample  260138      676.950 ± 36.243  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                           1  sample  276064      105.371 ±  1.859  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                           5  sample  255128      190.970 ± 16.475  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                          10  sample  272923      366.769 ± 28.204  ns/op
OutboundHeadersBenchmark.convertServerHeaders                                          20  sample  264797      641.961 ± 18.879  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                            1  sample  226078      425.262 ±  3.481  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                            5  sample  253606      675.488 ± 26.001  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                           10  sample  286556     1157.014 ± 12.923  ns/op
OutboundHeadersBenchmark.encodeClientHeaders                                           20  sample  345649     1874.806 ± 36.227  ns/op

Author: carl-mastrangelo

core/src/main/java/io/grpc/InternalMetadata.java

@@ -71,10 +71,27 @@ public interface TrustedAsciiMarshaller<T> {
   /**
    * Copy of StandardCharsets, which is only available on Java 1.7 and above.
    */
+  @Internal
   public static final Charset US_ASCII = Charset.forName("US-ASCII");
 
   @Internal
   public static <T> Key<T> keyOf(String name, TrustedAsciiMarshaller<T> marshaller) {
     return Metadata.Key.of(name, marshaller);
   }
+
+
+  @Internal
+  public static Metadata newMetadata(byte[]... binaryValues) {
+    return new Metadata(binaryValues);
+  }
+
+  @Internal
+  public static Metadata newMetadata(int usedNames, byte[]... binaryValues) {
+    return new Metadata(usedNames, binaryValues);
+  }
+
+  @Internal
+  public static byte[][] serialize(Metadata md) {
+    return md.serialize();
+  }
 }