Do not cache reflect.ValueOf() in metrics Labels

[jmacd]

There is a reflect.Value that is cached in the SDK labels type. This is a very minor optimization, having a ~5% impact on the performance of getting labels from a record in the export path. Removing this improves the code clarity.
This relates to #641

BEFORE:

BenchmarkLabelIterator-8   	60768566	        19.3 ns/op	       0 B/op	       0 allocs/op

AFTER:

BenchmarkLabelIterator-8   	58651670	        20.0 ns/op	       0 B/op	       0 allocs/op