prometheus plain text exporter

based on work done by Alberto Sartori <[email protected]>
in open-telemetry#619.

Author: RoadRunnr

apps/opentelemetry_experimental/include/otel_metrics.hrl

@@ -115,7 +115,7 @@
         {
          name :: otel_view:name(),
          scope :: opentelemetry:instrumentation_scope() | undefined,
-         description :: otel_instrument:description(),
-         unit :: otel_instrument:unit(),
+         description :: otel_instrument:description() | undefined,
+         unit :: otel_instrument:unit() | undefined,
          data :: #sum{} | #gauge{} | #histogram{}
         }).