tracing-core 0.1.27

This release of tracing-core introduces a new DefaultCallsite type, which
can be used by instrumentation crates rather than implementing their own
callsite types. Using DefaultCallsite may offer reduced overhead from callsite
registration.

Added

• DefaultCallsite, a pre-written Callsite implementation for use in
  instrumentation crates (#2083)
• ValueSet::len and Record::len methods returning the number of fields in a
  ValueSet or Record (#2152)

Changed

• Replaced lazy_static dependency with once_cell (#2147)

Documented

• Added documentation to the callsite module (#2088, #2149)

Thanks to new contributors @jamesmunns and @james7132 for contributing to this
release!