update the wording for callback based on PR comments

specification/metrics/new_api.md

@@ -355,13 +355,16 @@ some examples:
 * Return a list (or tuple, generator, enumerator, etc.) of `Measurement`s.
 * Use an observer argument to allow individual `Measurement`s to be reported.
 
-A single timestamp MUST be used per invocation of the callback. Duplicates are
-not allowed. If it happens, the [SDK](./README.md#sdk) can decide how to handle
-it. For example, during the callback invocation if two measurements `value=1,
-attributes={pid:4 bitness:64}` and `value=2, attributes={pid:4, bitness:64}` are
-reported, the SDK can decide to simply let them pass through (so the downstream
-consumer can handle duplication), drop the entire data, pick the last one, or
-something else.
+User code is recommended not to provide more than one `Measurement` with the
+same `attributes` in a single callback. If it happens, the
+[SDK](./README.md#sdk) can decide how to handle it. For example, during the
+callback invocation if two measurements `value=1, attributes={pid:4 bitness:64}`
+and `value=2, attributes={pid:4, bitness:64}` are reported, the SDK can decide
+to simply let them pass through (so the downstream consumer can handle
+duplication), drop the entire data, pick the last one, or something else. The
+API must treat observations from a single callback as logically taking place at
+a single instant, such that when recorded, observations from a single callback
+MUST be reported with identical timestamps.
 
 The API SHOULD provide some way to pass `state` to the callback. Individual
 language client can decide what is the idomatic approach (e.g. it could be an