v2.0.0-M5 - Minor cleanups

Changes since 2.0.0-M4:

This release packages a few minor changes required for modules to work as expected and simplifying reporter's implementation:

• Removed start method from metrics and span reporters. Long story short is that it lead to having lots of mutable stuff on reporters implementations, but you can read all details about it here: #588
• Remove all usages of scala.util.Try on the Kamon initialization path. This is because the latest futures instrumentation is targeting Try and it might happen these types will be initialized by Kamon before Kanela gets loaded (when attaching at runtime).
• Ignore negative values on gauges.