Add optional min and max fields to histogram data model.

specification/metrics/datamodel.md

@@ -386,6 +386,8 @@ Histograms consist of the following:
       (00:00:00 UTC on 1 January 1970).
   - A count (`count`) of the total population of points in the histogram.
   - A sum (`sum`) of all the values in the histogram.
+  - (optional) A min (`min`) representing the minimum value recorded.
+  - (optional) A max (`max`) representing the maximum value recorded.
   - (optional) A series of buckets with:
     - Explicit boundary values.  These values denote the lower and upper bounds
       for buckets and whether not a given observation would be recorded in this
@@ -398,6 +400,11 @@ denotes Delta temporality where accumulated event counts are reset to zero after
 and a new aggregation occurs. Cumulative, on the other hand, continues to
 aggregate events, resetting with the use of a new start time.
 
+The min and max fields behave differently based on aggregation temporality. When
+Delta, they represent the minimum and maximum values recorded over the time window. When
+Cumulative, they represent the minimum and maximum values recently recorded. The max age
+of recently recorded is commonly set to 10 minutes, and is not specified in the histogram.
+
 ### Summary (Legacy)
 
 [Summary](/~https://github.com/open-telemetry/opentelemetry-proto/blob/v0.9.0/opentelemetry/proto/metrics/v1/metrics.proto#L268)