Metrics SDK: don't create exemplar reservoirs when exemplars are disabled

[iRevive]

Currently, we create a reservoir and apply the filter afterward: 1) SumAggregator 2) ExplicitBucketHistogramAggregator

But there is no need to create a reservoir at all in the following scenarios: 1) If an exemplar filter is alwaysOff - use a noop reservoir 2) If an exemplar filter is traceBased and TraceContextLookup is Noop - use a noop reservoir