drpc: improve DRPC metadata context behavior for Add, Get

- The `Add` method should create a new context rather than manipulating the existing metadata of the context.
- Similarly, `Get` should return a copy of the metadata, not the entire original map itself.
- If we make the above change and `Add` creates a new context, then `AddPairs` should not use `Add`.

Reference: https://github.com/cockroachdb/drpc/blob/main/drpcmetadata/metadata.go

Jira issue: CRDB-53113