Recommend a default size of 1 for the SimpleFixedSizeExemplarReservoir

This reservoir type is used for all aggregations other than a histogram
with more than one bucket. Each attribute set the aggregation records
will have reservoir. Therefore, limiting this to a small value by
default when enabled is preferable.

This does not address the way a user will configure this value. That is
left for a future PR/Issue.

Part of open-telemetry#2421

specification/metrics/sdk.md

@@ -1033,7 +1033,8 @@ cycle. For the above example, that would mean that the `num_measurements_seen`
 count is reset every time the reservoir is collected.
 
 This Exemplar reservoir MAY take a configuration parameter for the size of the
-reservoir pool.
+reservoir pool. If no size configuration is provided, the default size of `1`
+SHOULD be used.
 
 #### AlignedHistogramBucketExemplarReservoir