Implement the keyed-attributes proposal

api/src/main/java/io/opentelemetry/common/AttributeConsumer.java

@@ -17,9 +17,10 @@
 package io.opentelemetry.common;
 
 /**
- * Convenience interface for consuming {@link ReadableAttributes}.
+ * Used for iterating over all the key/value pairs in an {@link Attributes} instance.
  *
  * @since 0.9.0
  */
-public interface AttributeConsumer
-    extends ReadableKeyValuePairs.KeyValueConsumer<String, AttributeValue> {}
+public interface AttributeConsumer {
+  <T> void consume(AttributeKey<T> key, T value);
+}

api/src/main/java/io/opentelemetry/common/AttributeKey.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2020, OpenTelemetry 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.opentelemetry.common;
+
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * This interface provides a handle for setting the values of {@link Attributes}. The type of value
+ * that can be set with an implementation of this key is denoted by the type parameter.
+ *
+ * <p>Implementations MUST be immutable, as these are used as the keys to Maps.
+ *
+ * @param <T> The type of value that can be set with the key.
+ */
+@SuppressWarnings("rawtypes")
+@Immutable
+public interface AttributeKey<T> extends Comparable<AttributeKey> {
+  /** Returns the underlying String representation of the key. */
+  @Nullable
+  String getKey();
+
+  /** Returns the type of attribute for this key. Useful for building switch statements. */
+  @Nonnull
+  AttributeType getType();
+}

api/src/main/java/io/opentelemetry/common/AttributeKeyImpl.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2020, OpenTelemetry 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.opentelemetry.common;
+
+import com.google.auto.value.AutoValue;
+
+@SuppressWarnings("rawtypes")
+@AutoValue
+abstract class AttributeKeyImpl<T> implements AttributeKey<T> {
+
+  static <T> AttributeKeyImpl<T> create(String key, AttributeType type) {
+    return new AutoValue_AttributeKeyImpl<>(key, type);
+  }
+
+  //////////////////////////////////
+  // IMPORTANT: the equals/hashcode/compareTo *only* include the key, and not the type,
+  // so that de-duping of attributes is based on the key, and not also based on the type.
+  //////////////////////////////////
+
+  @Override
+  public final boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (!(o instanceof AttributeKeyImpl)) {
+      return false;
+    }
+
+    AttributeKeyImpl<?> that = (AttributeKeyImpl<?>) o;
+
+    return getKey() != null ? getKey().equals(that.getKey()) : that.getKey() == null;
+  }
+
+  @Override
+  public final int hashCode() {
+    return getKey() != null ? getKey().hashCode() : 0;
+  }
+
+  @Override
+  public int compareTo(AttributeKey o) {
+    if (getKey() == null) {
+      return o.getKey() == null ? 0 : -1;
+    }
+    if (o.getKey() == null) {
+      return 1;
+    }
+    return getKey().compareTo(o.getKey());
+  }
+}

api/src/main/java/io/opentelemetry/common/AttributeType.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2020, OpenTelemetry 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.opentelemetry.common;
+
+/**
+ * An enum that represents all the possible value types for an {@code AttributeKey} and hence the
+ * types of values that are allowed for {@link Attributes}.
+ *
+ * @since 0.1.0
+ */
+public enum AttributeType {
+  STRING,
+  BOOLEAN,
+  LONG,
+  DOUBLE,
+  STRING_ARRAY,
+  BOOLEAN_ARRAY,
+  LONG_ARRAY,
+  DOUBLE_ARRAY
+}