Add notes about MetricExporter naming (#2286)

If the implementors choose to implement pull metric exporters modeled as a metric reader, the only variant for metric exporter interface is push metric exporter. While the pull metric exporter is modeled as a "metric reader", the better naming for the genric "metric exporter" interface may be "push metric exporter".

Just open this PR to see if there are similar naming confusions with other SDK implementors. If so, I'd believe the metric exporter and metric reader section need a re-structure in a follow-up PR since how the pull metric exporter is modeled is important to the naming of the metric exporter interface.

Related:
- open-telemetry/opentelemetry-js#2707
- open-telemetry/opentelemetry-js#2589 (comment)

specification/metrics/sdk.md

@@ -876,7 +876,9 @@ Implementors MAY choose the best idiomatic design for their language. For
 example, they could generalize the [Push Metric Exporter
 interface](#push-metric-exporter) design and use that for consistency, they
 could model the pull exporter as [MetricReader](#metricreader), or they could
-design a completely different pull exporter interface.
+design a completely different pull exporter interface. If the pull exporter is
+modeled as MetricReader, implementors MAY name the MetricExporter interface as
+PushMetricExporter to prevent naming confusion.
 
 The following diagram gives some examples on how `Pull Metric Exporter` can be
 modeled to interact with other components in the SDK: