core: add accessor for perfmark (#5687)

* core: add accessor for perfmark

* core: update BUILD.bazel to exclude internal classes

core/BUILD.bazel

@@ -1,3 +1,16 @@
+PERFMARK_INTERNAL_ACCESSOR_SRCS = glob(
+    [
+        "src/main/java/io/grpc/perfmark/Internal*.java",
+    ],
+)
+
+PERFMARK_SRCS = glob(
+    [
+        "src/main/java/io/grpc/perfmark/*.java",
+    ],
+    exclude = PERFMARK_INTERNAL_ACCESSOR_SRCS,
+)
+
 java_library(
     name = "core",
     visibility = ["//visibility:public"],
@@ -65,11 +78,7 @@ java_library(
 
 java_library(
     name = "perfmark",
-    srcs = glob(
-        [
-            "src/main/java/io/grpc/perfmark/*.java",
-        ],
-    ),
+    srcs = PERFMARK_SRCS,
     visibility = ["//:__subpackages__"],
     deps = [
         "@com_google_code_findbugs_jsr305//jar",

core/src/main/java/io/grpc/perfmark/InternalPerfMark.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2019 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc.perfmark;
+
+
+/**
+ * Internal {@link PerfTag.TagFactory} and {@link PerfMarkTask} accessor. This is intended for use
+ * by io.grpc.perfmark, and the specifically supported packages that utilize PerfMark. If you
+ * *really* think you need to use this, contact the gRPC team first.
+ */
+public final class InternalPerfMark {
+
+  private InternalPerfMark() {}
+
+  /** Expose class to allow packages that utilize PerfMark to get PerfMarkTask instances. */
+  public abstract static class InternalPerfMarkTask extends PerfMarkTask {
+    public InternalPerfMarkTask() {}
+  }
+
+  /** Expose methods that create PerfTag to packages that utilize PerfMark. */
+  private static final long NULL_NUMERIC_TAG = 0;
+  private static final String NULL_STRING_TAG = null;
+
+  public static PerfTag createPerfTag(long numericTag, String stringTag) {
+    return PerfTag.TagFactory.create(numericTag, stringTag);
+  }
+
+  public static PerfTag createPerfTag(String stringTag) {
+    return PerfTag.TagFactory.create(NULL_NUMERIC_TAG, stringTag);
+  }
+
+  public static PerfTag createPerfTag(long numericTag) {
+    return PerfTag.TagFactory.create(numericTag, NULL_STRING_TAG);
+  }
+}