Introduce "split" metric schema transformation (#2653)

This is a new transformation type that allows to describe a change
where a metric is converted to several other metrics by eliminating
an attribute.

An example of such change that happened recently is this:
#2617

This PR contains:
- A new "split" transformation added to the schema file format.
- Schema file format incremented from 1.0.0 to 1.1.0. This is considered
  a backward compatible change and thus the minor version number is
  incremented.
- Deleted incorrect sentence that claimed that the addition of a new
  transformation type is not backward compatible and requires a
  major version number change. This is incorrect. Addition of
  new transformation types is backward compatible according to our
  current definition of backward compatibility, which is:
  "consumers can consume version X.Y and older versions X.Z
  provided that they are aware of version X.Y".
- Deleted the requirement that a full OTEP is always necessary to
  introduce new transformation types. This seems excessive. I think
  in simple cases like this a PR directly in the spec is sufficient.

This draft PR shows how the corresponding implementation of schema file
parser in Go can be done:
open-telemetry/opentelemetry-go#2999

This commit in my personal repo shows in more detail how the "split"
transformation can be implemented:
tigrannajaryan/telemetry-schema@2b45e48

CHANGELOG.md

@@ -72,6 +72,9 @@ release.
 
 ### Telemetry Schemas
 
+- Introduce "split" metric schema transformation
+  ([#2653](https://github.com/open-telemetry/opentelemetry-specification/pull/2653)).
+
 ### Common
 
 - Introduce Instrumentation Scope Attributes