Introduce RequestLogListener

Motivation:

A single `RequestLogProperty` can be received through
`RequestLog.whenAvailable()`, but it is not straightforward to receive
all events published by `RequestLog` using `.whenAvailable()`.

To address the limitation, I propose introducing an interface that
listens to all events by adding it to `RequestLog`. This will also
simplify the integration of other implmentations used for collecting
metrics.

Motifications:

- Introduced `RequestLogListener` API that can be attached to
  `RequestLog`.
  - `RequestLogAccess.addListener()` API was added and
    `DefaultRequestLog` implmemeted it.
  - The listener will be notified whenever a new property is set to
    `RequestLog`. If some properties have already been set, they will
    notified of them immediately.
- Add `REQUEST_COMPLETE`, `RESPONSE_COMPLETE` and `ALL_COMPLETE` to
  `RequestLogProperty`.
  - Previously, there were APIs such as `whenRequestComplete()` and
    `whenComplete()` that computed and signaled request or response
    completion and but no explicit properties exists for them.
    `RequestLogProperty` should represent all states in
    `RequestLogListener`, I added the new completion properties.
- Simplied child log propagation in `DefaultRequestLog` and
  the `Observation{Service,Client} implementations by using
  `RequestLogListener`.

Result:

You can now use `RequestLogListener` to observe all `RequestLog` events.

Author: ikhoon

core/src/main/java/com/linecorp/armeria/client/observation/ObservationClient.java

@@ -29,7 +29,6 @@
 import com.linecorp.armeria.common.RequestHeadersBuilder;
 import com.linecorp.armeria.common.annotation.Nullable;
 import com.linecorp.armeria.common.annotation.UnstableApi;
-import com.linecorp.armeria.common.logging.RequestLogProperty;
 import com.linecorp.armeria.internal.common.RequestContextExtension;
 import com.linecorp.armeria.server.observation.ObservationService;
 
@@ -138,24 +137,26 @@ public HttpResponse execute(ClientRequestContext ctx, HttpRequest req) throws Ex
     private static void enrichObservation(ClientRequestContext ctx,
                                           ClientObservationContext clientObservationContext,
                                           Observation observation) {
-        ctx.log()
-           .whenAvailable(RequestLogProperty.REQUEST_FIRST_BYTES_TRANSFERRED_TIME)
-           .thenAccept(requestLog -> observation.event(Events.WIRE_SEND));
-
-        ctx.log()
-           .whenAvailable(RequestLogProperty.RESPONSE_FIRST_BYTES_TRANSFERRED_TIME)
-           .thenAccept(requestLog -> {
-               if (requestLog.responseFirstBytesTransferredTimeNanos() != null) {
-                   observation.event(Events.WIRE_RECEIVE);
-               }
-           });
-
-        ctx.log().whenComplete()
-           .thenAccept(requestLog -> {
-               // TODO: ClientConnectionTimings - there is no way to record events
-               // with a specific timestamp for an observation
-               clientObservationContext.setResponse(requestLog);
-               observation.stop();
-           });
+        ctx.log().addListener((property, log) -> {
+            switch (property) {
+                case REQUEST_FIRST_BYTES_TRANSFERRED_TIME:
+                    observation.event(Events.WIRE_SEND);
+                    break;
+                case RESPONSE_FIRST_BYTES_TRANSFERRED_TIME:
+                    if (log.responseFirstBytesTransferredTimeNanos() != null) {
+                        observation.event(Events.WIRE_RECEIVE);
+                    }
+                    break;
+                case ALL_COMPLETE:
+                    // TODO: ClientConnectionTimings - there is no way to record events
+                    // with a specific timestamp for an observation
+                    clientObservationContext.setResponse(log);
+                    observation.stop();
+                    break;
+                default:
+                    // Do nothing.
+                    break;
+            }
+        });
     }
 }