log: make span enter/exit targets always TRACE

[hawkw]

Motivation

Currently, tracing's log feature emits log records for creating,
entering, exiting, and dropping a Span at the log equivalent of that
span's tracing level. If the span has any fields, the span creation
logs are sent to the same target that the span would have, since the
fields provide high-value information that users enabling that target
will probably want to see. However, the other events for entering,
exiting, and dropping a span do not include fields, and are much more
verbose, so they are sent to a special tracing::span target. This
allows them to be filtered independently, and only enabled when needed.

However, some log users have noted that when using blanket log
filters, like setting the level to info globally, the more verbose
activity logs will still be enabled for spans whose level matches. This
can be annoying.

Solution

Therefore, this branch changes the span activity and lifecycle logs to
always have the Trace log level, even when the span that generated
them has a less-verbose level. When fields are recorded, the emitted log
records are sent to the span's target instead, since users probably want
this information if they have enabled the span's level for its target.

Additionally, I've added a sub-target of tracing::span for the
enter/exit logs in particular (tracing::span::active). This allows
them to be filtered independently of the less frequent creation/drop
logs.

Signed-off-by: Eliza Weisman eliza@buoyant.io