motivation: seperate gauge from recorder in a backwards compatible way
changes:
* add new meter and meter handler pair
* add increment and decrement to meter handler
* add default implementation based on recorder for backwards compatibility
* add and adjust tests
motivation: adjust to swift 5.6
changes:
* define sendable shims for protocols and structs that may be used in async context
* refactor Gauge to include rather than inherit Recorder
* adjust tests
* add a test to make sure no warning are emitted
hamzahrmalik
tomer doron
Fabian Fett
Konrad `ktoso` Malawski