Add recommendation to use non-prefixed units for metric instruments

CHANGELOG.md

@@ -19,6 +19,8 @@ release.
 
 - Clarify that units should use UCUM case sensitive variant.
   ([#3306](https://github.com/open-telemetry/opentelemetry-specification/pull/3306))
+- Recommended non-prefixed units for metric instrument semantic conventions.
+  ([#3312](https://github.com/open-telemetry/opentelemetry-specification/pull/3312))
 - Remove No-Op instrument and Meter creation requirements.
   ([#3322](https://github.com/open-telemetry/opentelemetry-specification/pull/3322))
 - Fixed attributes requirement level in semantic conventions for hardware metrics

specification/metrics/semantic_conventions/README.md

@@ -190,6 +190,12 @@ give additional meaning *without* the leading default unit (`1`). For example,
 use `{packet}`, `{error}`, `{fault}`, etc.
 - Instrument units SHOULD be specified using the UCUM case sensitive ("c/s")
   variant. For example, "Cel" for the unit with full name "degree Celsius".
+- Instruments SHOULD use non-prefixed units (i.e. `By` instead of `MiBy`)
+  unless there is good technical reason to not do so.
+  - If using a non-prefixed unit would limit the data that could be
+    represented, an instrument SHOULD use an appropriately prefixed unit. For
+    example, an instrument measuring a duration of time as an integer that
+    needs a precision of nanoseconds should use a unit of `ns` instead of `s`.
 
 ### Instrument Types