`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
Sleuth is unlike most tracing configuration libraries, as it has a
legacy from 1.x as being primarily log correlation. In short, when
Zipkin is not installed, it ignored the sampling properties. This
behavior was managed implicitly through an untested combination of
configuration conventions between the core and zipkin modules.
16fb8e3 broke this and this change puts it back, in a way not tightly
coupled to Zipkin and neither requires the more simple, but confusing
"import SamplerAutoConfiguration" approach. It also backfills tests
that should have broken earlier.
In practice, a site that only uses logging is likely uniform in that, so
whether or not the sampled bit is set is of no consequence. However,
there's a chance that someone might rely on this historical behavior.
We should follow-up on 3.0 and remove this as it is very unintuitive to
intentionally ignore sampling properties. For now, this restores the old
behavior based on heuristics of bean definitions.
