[improvement] Adds key traceSampled to MDC when set trace is observab…

…le (#144)

## Before this PR
Log consumers were unable to tell which traces were sampled (observable: true) vs which traces were not sampled (observable: false).

## After this PR
An MDC key traceSampled will now appear with the value "1" when the set trace has been sampled (observable: true). If the set trace is not sampled (observable: false), the key will not appear in the MDC.

Note: this replaces #104

tracing/src/main/java/com/palantir/tracing/Tracer.java

@@ -338,8 +338,19 @@ static Optional<Trace> copyTrace() {
     static void setTrace(Trace trace) {
         currentTrace.set(trace);
 
-        // Give SLF4J appenders access to the trace id
+        // Give log appenders access to the trace id and whether the trace is being sampled
         MDC.put(Tracers.TRACE_ID_KEY, trace.getTraceId());
+        setTraceSampledMdcIfObservable(trace.isObservable());
+    }
+
+    private static void setTraceSampledMdcIfObservable(boolean observable) {
+        if (observable) {
+            // Set to 1 to be consistent with values associated with http header key TraceHttpHeaders.IS_SAMPLED
+            MDC.put(Tracers.TRACE_SAMPLED_KEY, "1");
+        } else {
+            // To ensure MDC state is cleared when trace is not observable
+            MDC.remove(Tracers.TRACE_SAMPLED_KEY);
+        }
     }
 
     private static Trace getOrCreateCurrentTrace() {
@@ -354,5 +365,6 @@ private static Trace getOrCreateCurrentTrace() {
     private static void clearCurrentTrace() {
         currentTrace.remove();
         MDC.remove(Tracers.TRACE_ID_KEY);
+        MDC.remove(Tracers.TRACE_SAMPLED_KEY);
     }
 }

tracing/src/main/java/com/palantir/tracing/Tracers.java

@@ -26,6 +26,10 @@
 public final class Tracers {
     /** The key under which trace ids are inserted into SLF4J {@link org.slf4j.MDC MDCs}. */
     public static final String TRACE_ID_KEY = "traceId";
+    /** The key under which trace sampling state are inserted into SLF4J {@link org.slf4j.MDC MDCs}. If present, the
+     * field can take the values of "1" or "0", where "1" indicates the trace was sampled.
+     */
+    public static final String TRACE_SAMPLED_KEY = "_sampled";
     private static final String DEFAULT_ROOT_SPAN_OPERATION = "root";
     private static final char[] HEX_DIGITS =
             {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

tracing/src/test/java/com/palantir/tracing/TracerTest.java

@@ -190,6 +190,22 @@ public void testSetTraceSetsCurrentTraceAndMdcTraceIdKey() throws Exception {
         assertThat(MDC.get(Tracers.TRACE_ID_KEY)).isNull();
     }
 
+    @Test
+    public void testSetTraceSetsMdcTraceSampledKeyWhenObserved() {
+        Tracer.setTrace(new Trace(true, "observedTraceId"));
+        assertThat(MDC.get(Tracers.TRACE_SAMPLED_KEY)).isEqualTo("1");
+        assertThat(Tracer.completeSpan()).isEmpty();
+        assertThat(MDC.get(Tracers.TRACE_SAMPLED_KEY)).isNull();
+    }
+
+    @Test
+    public void testSetTraceMissingMdcTraceSampledKeyWhenNotObserved() {
+        Tracer.setTrace(new Trace(false, "notObservedTraceId"));
+        assertThat(MDC.get(Tracers.TRACE_SAMPLED_KEY)).isNull();
+        assertThat(Tracer.completeSpan()).isEmpty();
+        assertThat(MDC.get(Tracers.TRACE_SAMPLED_KEY)).isNull();
+    }
+
     @Test
     public void testCompletedSpanHasCorrectSpanType() throws Exception {
         for (SpanType type : SpanType.values()) {