Audio's callerId depends on the user name and there isn't
an "on-demand" way of fetching that field internally, making callerId
assembly with trusted attributes (server-side generated) impossible in
bbb-webrtc-sfu.
The new extra header (User-Name, mapped to user_name in the proxied
connection) allows fetching the user name field in a cheap way and
consequently provides a cheap+safe way of assembling the callerId.
Alternatives I've considered but discarded:
- a new akka-apps req-resp pair for fetching the user name (+overhead)
- a new akka-apps req-resp pair for generating the callerId (+overhead)
- piggybacking on GetMicrophonePermissionReq/Resp to generate the
callerId (same overhead, but mixing responsabilities)
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