Controller's `/segments/<tableName>/metadata` does not return metadata from all segments of the table

**Following Slack discussion:** https://apache-pinot.slack.com/archives/C011C9JHN7R/p1625240658371100

This endpoint is supposed (see Swagger screenshot) to return the metadata of all segments. However, it appears to only return a single segment's metadata (from a "random one").

The table really has many segments, and multiple calls to this endpoint return metadata from different segments every time.

**Expected**

**Actual**
https://user-images.githubusercontent.com/22521284/125203010-c1c71480-e276-11eb-8a60-fd2e901d9983.mp4

Start at the controller implementation for `/segments//metadata` and reproduce the endpoint with a table containing many segments. Compare repeated responses with the expected Swagger behavior and verify that the result includes metadata from all segments rather than one varying segment. Done means the endpoint consistently returns complete metadata for every segment.

Written by the indexing model from the issue text.