[profiles] standardize and document the concept of SampleType #794

The SampleType of a Sample is defined here: https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/profiles/v1development/profiles.proto#L289

  // The type and unit of all Sample.values in this profile.
  // For a cpu or off-cpu profile this might be:
  //   ["cpu","nanoseconds"] or ["off_cpu","nanoseconds"]
  // For a heap profile, this might be:
  //   ["allocated_objects","count"] or ["allocated_space","bytes"],
  ValueType sample_type = 1;

The type is an open ended field at this time.

What is a valid value for the type of samples? Is this field open ended or meant to be an enumeration?
Is the unit important to the type and should it be defined as part of the SampleType?

Is there a definition for the acceptable values of types, such as cpu, allocation, etc?