Fix confusing double usage of 'metadata'

What does this MR do?

Renames a variable and improves method documentation.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

There was a confusing double use of 'metadata' in GitalyClient.call. The function yielded a block argument called 'metadata', which would itself be a hash { metadata: { 'foo' => 'bar' }. This change renames the outer variable to 'kwargs'.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?