specify that the callback ensures a single timestamp

specification/metrics/new_api.md

@@ -351,11 +351,12 @@ 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.
 
-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 drop the
-entire data, pick the last one, take all the data and add them together, or
+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.
 
 The API SHOULD provide some way to pass `state` to the callback. Individual