Make zipkin's current context can be nested

Apply this change will fix TraceContext leak in thread problem.
There is a case that we keep armeria's request context aware CompletableFuture
for fetching data, and let multiple request register callback on that future.
So there is a problem that future uses first request's eventloop and RequestContext
to call waiting handlers, but each handler may wrap with it's RequestContext, so
there is nested RequestContext on same thread(onEnter->onEnter->onExit->onExit).

Another solution is application developer should always jump back to current request's
context aware eventloop. e.g.

```

cachedFuture.acceptAsync((r, e)->{
  ...
}, RequestContext.current.contextAwareEventloop);

```

I'm not sure if armeria side need to take care of this or not...

zipkin/src/main/java/com/linecorp/armeria/client/tracing/HttpTracingClient.java

@@ -33,6 +33,7 @@
 import com.linecorp.armeria.common.logging.RequestLogAvailability;
 import com.linecorp.armeria.internal.tracing.AsciiStringKeyFactory;
 import com.linecorp.armeria.internal.tracing.SpanContextUtil;
+import com.linecorp.armeria.internal.tracing.SpanInScopeWrapper;
 
 import brave.Span;
 import brave.Span.Kind;
@@ -52,7 +53,7 @@
  */
 public class HttpTracingClient extends SimpleDecoratingClient<HttpRequest, HttpResponse> {
 
-    private static final FastThreadLocal<SpanInScope> SPAN_IN_THREAD = new FastThreadLocal<>();
+    private static final FastThreadLocal<SpanInScopeWrapper> SPAN_IN_THREAD = new FastThreadLocal<>();
 
     /**
      * Creates a new tracing {@link Client} decorator using the specified {@link Tracing} instance.

zipkin/src/main/java/com/linecorp/armeria/internal/tracing/SpanContextUtil.java

@@ -32,13 +32,23 @@ public final class SpanContextUtil {
     /**
      * Sets up the {@link RequestContext} to push and pop the {@link Span} whenever it is entered/exited.
      */
-    public static void setupContext(FastThreadLocal<SpanInScope> threadLocalSpan, RequestContext ctx, Span span,
-                                    Tracer tracer) {
-        ctx.onEnter(unused -> threadLocalSpan.set(tracer.withSpanInScope(span)));
+    public static void setupContext(FastThreadLocal<SpanInScopeWrapper> threadLocalSpan, RequestContext ctx,
+                                    Span span, Tracer tracer) {
+        ctx.onEnter(unused -> {
+            final SpanInScopeWrapper current = threadLocalSpan.get();
+            final SpanInScope newScope = tracer.withSpanInScope(span);
+            threadLocalSpan.set(new SpanInScopeWrapper(newScope, current));
+        });
         ctx.onExit(unused -> {
-            final SpanInScope spanInScope = threadLocalSpan.get();
-            if (spanInScope != null) {
-                spanInScope.close();
+            final SpanInScopeWrapper spanInScope = threadLocalSpan.get();
+            if (spanInScope == null) {
+                return;
+            }
+            spanInScope.close();
+            final SpanInScopeWrapper previousScope = spanInScope.getPrevious();
+            if (previousScope != null) {
+                threadLocalSpan.set(previousScope);
+            } else {
                 threadLocalSpan.remove();
             }
         });

zipkin/src/main/java/com/linecorp/armeria/internal/tracing/SpanInScopeWrapper.java

@@ -0,0 +1,31 @@
+package com.linecorp.armeria.internal.tracing;
+
+import javax.annotation.Nullable;
+
+import brave.Tracer.SpanInScope;
+
+/**
+ * SpanInScope Wrapper for keeping previous span scope.
+ */
+public final class SpanInScopeWrapper implements AutoCloseable {
+
+    private final SpanInScope spanInScope;
+
+    @Nullable
+    private final SpanInScopeWrapper previous;
+
+    public SpanInScopeWrapper(SpanInScope current, @Nullable SpanInScopeWrapper previous) {
+        this.spanInScope = current;
+        this.previous = previous;
+    }
+
+    @Nullable
+    public SpanInScopeWrapper getPrevious() {
+        return previous;
+    }
+
+    @Override
+    public void close() {
+        spanInScope.close();
+    }
+}

zipkin/src/main/java/com/linecorp/armeria/server/tracing/HttpTracingService.java

@@ -24,6 +24,7 @@
 import com.linecorp.armeria.common.logging.RequestLogAvailability;
 import com.linecorp.armeria.internal.tracing.AsciiStringKeyFactory;
 import com.linecorp.armeria.internal.tracing.SpanContextUtil;
+import com.linecorp.armeria.internal.tracing.SpanInScopeWrapper;
 import com.linecorp.armeria.server.Service;
 import com.linecorp.armeria.server.ServiceRequestContext;
 import com.linecorp.armeria.server.SimpleDecoratingService;
@@ -46,7 +47,7 @@
  */
 public class HttpTracingService extends SimpleDecoratingService<HttpRequest, HttpResponse> {
 
-    private static final FastThreadLocal<SpanInScope> SPAN_IN_THREAD = new FastThreadLocal<>();
+    private static final FastThreadLocal<SpanInScopeWrapper> SPAN_IN_THREAD = new FastThreadLocal<>();
 
     /**
      * Creates a new tracing {@link Service} decorator using the specified {@link Tracing} instance.