Clarify meaning of *DNS name prefix* in protobuf code comment for metric name

[sebastien-rosset]

What are you trying to achieve?

Improve the code comments for the metric name in the protobuf spec.

What did you expect to see?

I was expecting the meaning of the DNS name prefix to be explained in the protobuf spec or in the OpenTelemetry specification.

In the protobuf spec, the name of a Metric is documented as:

name of the metric, including its DNS name prefix. It must be unique.

However, I couldn't find the meaning of DNS name prefix, either in the protobuf spec or in the OpenTelemetry specification. If it's referring to the Internet DNS (RFC 1034) or maybe overloading the word "DNS" to mean the hierarchical naming guidelines for metrics.

Additional context.