Expose merge time metrics on GraphQL flowMetrics

Merge time virtual stage was added with !117986 (merged) now we need to expose it on GraphQL flowMetrics node.

The following discussion from !117986 (merged) should be addressed:

• @felipe_artur started a discussion: (+1 comment)

  @cablett @ahegyi Do we need a follow-up to add time to merge stage into GraphQL flowMetrics?