Add a note about recording with extra tags. (#145)

* Add a note about recording with extra tags.

* Add a code example to reflect this change.

* Split the code examples.

* Slightly reword on non-propagating.

Author: songy23

stats/Record.md

@@ -51,7 +51,10 @@ contextual information of an exemplar, for example trace id, span id or dropped
 ## Recording Stats
 
 Users should record Measurements against a context, either an explicit context or the implicit 
-current context. Tags from the context are recorded with the Measurements if they are any.
+current context. Tags from the context are recorded with the Measurements if they are any. 
+When recording against an explicit context, implmentations should allow users to add extra tags,
+and those tags should not be added to current context..
+
 Note that there is no implicit recording for exemplars. If you want to record a `Measurement`
 against an exemplar, you have to explicitly pass a string-string map.
 
@@ -64,16 +67,33 @@ argument. e.g. `record(List<Measurement>)` or `record(...Measurement)`.
 Example in Java
 
 ```java
+// Static constants.
 private static final MeasureDouble RPC_LATENCY =
     MeasureDouble.create("grpc.io/client/latency", "latency", "ms");
 private static final MeasureLong RPC_BYTES_SENT =
     MeasureLong.create("grpc.io/client/bytes_sent", "bytes sent", "kb");
+private static final TagKey MY_KEY = TagKey.create("my.org/key");
+```
 
+```java
+// Record against implicit context.
 MeasurementMap measurementMap = new MeasurementMap();
 measurementMap.put(RPC_LATENCY, 10.3);
 measurementMap.put(RPC_BYTES_SENT, 124);
 measurementMap.record();  // reads context from thread-local.
+```
 
+```java
+// Record against explicit context.
+MeasurementMap measurementMap = new MeasurementMap();
+measurementMap.put(RPC_LATENCY, 15);
+measurementMap.put(RPC_BYTES_SENT, 200);
+TagValue value = TagValue.create("some value");
+measurementMap.record(
+    Tags.getTagger().currentBuilder().put(MY_KEY, value).build());  // record against an extra tag.
+```
+
+```java
 // Another example on recording against sampled SpanContext.
 SpanContext spanContext = tracer.getCurrentSpan().getContext();
 if (spanContext.getTraceOptions().isSampled()) {