Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

See other samples in MapFeatureExtractorSamples (e.g. valuesFeature and values) we want the same for the function size

• add sizeFeature and size to MapFeatureExtractorSamples in api-fluent
• add @sample to the corresponding functions in mapFeatureExtractors.kt in api-fluent
• add sizeFeature and size to MapFeatureExtractorSamples in api-infix
• add @sample to the corresponding functions in mapFeatureExtractors.kt in api-infix

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