Rename "labels" to "attributes" OTEP 0152

[tigrannajaryan]

Fixes https://github.com/open-telemetry/opentelemetry-specification/issues/1989

OTEP 0152 predates renaming of labels to attributes in the spec. Unfortunately the schema file format still uses the term "label" in the metrics section this way:

    metrics:
      changes:
        - rename_labels:
            label_map:
              # map of key/values. The keys are the old label name used
              # in the previous version, the values are the new label name
              # starting from this version.

            apply_to_metrics:
              # Optional. If it is missing the transformation is applied
              # to all metrics. If it is present the transformation is applied
              # only to the metrics with the name that is found in the sequence
              # specified below.

We don't want the word label in the schema files, instead we want the word attribute to be used instead.

Discussed in spec meeting, decided:

• OTEPs which are not part of the spec repo are not considered to be final, or be an actual part of stable spec (OTEPs are not labeled "stable" in any way).
• Modify the OTEP, rename label to attribute (this commit).
• Merge the OTEP with the spec.

[tigrannajaryan]

@open-telemetry/specs-metrics-approvers @open-telemetry/specs-approvers please review.

[tigrannajaryan]

I need 2 more approvals, please.

[tigrannajaryan]

https://github.com/open-telemetry/opentelemetry-go/pull/2267 is blocked on this.