Brave's SpanHandler can report natively in other formats which have different
constraints than Zipkin and often extensions to the data model.
This change ports all tests away from Zipkin's types so that it is more clear
what's actually recorded vs what's a side-effect of Zipkin conversion.
This removes `BlockingQueueSpanReporter` which was never released, also.
`SpanHandler` is the base type for the now deprecated `FinishedSpanHandler`.
Notable, it can not just handle things at the end of a recording, but also the
beginning.
For example, this permits set-once baggage without the HTTP abstraction:
```java
static final BaggageField EPOCH_SECONDS = BaggageField.create("epoch_seconds");
static final class RootOnlyBaggage extends SpanHandler {
@Override
public boolean begin(TraceContext context, MutableSpan span, @Nullable TraceContext parent) {
if (EPOCH_SECONDS.getValue(context) == null) { // only set at the first span
long epochSeconds = System.currentTimeMillis() / 1000;
EPOCH_SECONDS.updateValue(context, String.valueOf(epochSeconds));
}
return true;
}
@Override public boolean end(TraceContext context, MutableSpan span, Cause cause) {
Tags.BAGGAGE_FIELD.tag(EPOCH_SECONDS, context, span);
return true;
}
}
```
As the parent is available, it can also facilitate advanced tasks like counting
children, or summarizing entire local roots.
See https://github.com/openzipkin/brave/tree/master/brave/src/test/java/brave/features/handler
and https://github.com/openzipkin/brave/blob/master/brave/src/main/java/brave/handler/SpanHandler.java for more
To reduce confusion and overhead, the following custom spring-messaging headers added in Sleuth 1.0 are no longer sent, and a log warning is issued once if they are by outside code.
* spanId
* spanSampled
* spanParentSpanId
* spanTraceId
* spanFlags
Sending the above headers actually increases the headers by up to 10 because they are duplicated in the "native" part of messages. This overhead is extreme especially if messages never leave the process.
The solution is to only send [b3 single format](https://github.com/openzipkin/b3-propagation#single-header), which has been in sleuth since 2.0 and is compatible with JMS. The B3 single format is always parsed and takes precedence, even if multiple headers are sent, so this is a safe change.
Note: Unlike RPC, messaging spans never join with their parent. Better performance is achieved by not propagating the producer's parentId downstream.
Note: Deprecated spring-messaging headers such "spanTraceId" as are still read in Sleuth 3.x. However, they will not be at some point in the future. Please pay attention to the log messages and update your code if you are accidentally using them.
