Splunk HEC metrics mapping misses support for summaries

[atoulme]

The Splunk HEC metrics mapping takes OpenTelemetry native metrics and transfers them into domain objects to be serialized to JSON to be sent to Splunk.
The hec mapping is in the metricdata_to_splunk.go file. The mapping lists explicitly each of the supported metric types.

The summary metric type is currently not mapped, and therefore those metrics are not sent to Splunk.
The collector logs a warning and moves on.

Describe the solution you'd like
A mapping for summaries.