Rename observer result to observable result (open-telemetry#2238)

Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

Author: legendecas

specification/metrics/api.md

@@ -416,7 +416,7 @@ function(s) independently.
 approach. Here are some examples:
 
 * Return a list (or tuple, generator, enumerator, etc.) of `Measurement`s.
-* Use an observer result argument to allow individual `Measurement`s to be
+* Use an observable result argument to allow individual `Measurement`s to be
   reported.
 
 User code is recommended not to provide more than one `Measurement` with the
@@ -628,7 +628,7 @@ function(s) independently.
 approach. Here are some examples:
 
 * Return a list (or tuple, generator, enumerator, etc.) of `Measurement`s.
-* Use an observer result argument to allow individual `Measurement`s to be reported.
+* Use an observable result argument to allow individual `Measurement`s to be reported.
 
 User code is recommended not to provide more than one `Measurement` with the
 same `attributes` in a single callback. If it happens, the
@@ -905,7 +905,7 @@ function(s) independently.
 approach. Here are some examples:
 
 * Return a list (or tuple, generator, enumerator, etc.) of `Measurement`s.
-* Use an observer result argument to allow individual `Measurement`s to be
+* Use an observable result argument to allow individual `Measurement`s to be
   reported.
 
 User code is recommended not to provide more than one `Measurement` with the