Moved repeated labels to Labels message.

We suggested to change hooks into manipulating task labels only. With
that in mind and based on BenH's suggestion originally, this patch set
wraps the repeated Labels field in a message when can be used without
referring to repeated field types.

Review: https://reviews.apache.org/r/28574
Changed task to use Labels.

Review: https://reviews.apache.org/r/28575
Changed HTTP task modeling to use Labels.

Review: https://reviews.apache.org/r/28576
Changed tests to use Labels.

Review: https://reviews.apache.org/r/28577