Add an InitializeMetricSlice (or similar) function to pdatautil package

[james-bebbington]

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Add a convenience method for initializing a metric with a single resource & instrumentation metric to the pdatautil package. Something like:

func InitializeMetricSlice(metricData data.MetricData) pdata.MetricSlice (may need additional parameters)

Once we have this we should update the host metrics receiver to make use of it.

Originally posted by @bogdandrutu in https://github.com/open-telemetry/opentelemetry-collector/pull/862