Add support for measurements

[mszczygiel]

Hi, it would be great to have a support for measurements (as per https://opencensus.io/stats/measurement/) which is supported in Erlang library already (https://hexdocs.pm/opencensus/oc_stat_measure.html). I would be happy to contribute it if it's something you would like to have in the library.

[hauleth]

We are open for PR, however the near future we probably will move to the OpenTelemetry