Add timestamps to TranscriptionSegment (#459)

This adds tracking of transcription receipt times which simplifies the
consumption of this feature. I also removed startTime and endTime from
the version we expose, since neither is used yet at the protocol level.

The new fields are `firstReceivedTime` and `lastReceivedTime`.

I tried this out in my sample project and it really simplified things.
Now I can implement a very basic real-time chat thread without needing
multiple data structures or new types. The only thing that still
requires extra bookkeeping is tracking participant name, etc, since
TranscriptionSegment lives under TrackPublication and Participant, and
we don't denormalize the Participant id down the chain.

See the [associated commit in my docs
PR](livekit/web@0076c75)
for a visual of how this simplifies basic uses

Sources/LiveKit/Core/Room+EngineDelegate.swift

@@ -207,13 +207,30 @@ extension Room {
             return
         }
 
-        let segments = packet.segments.map { $0.toLKType() }
-
-        guard !segments.isEmpty else {
+        guard !packet.segments.isEmpty else {
             log("[Transcription] Received segments are empty", .warning)
             return
         }
 
+        let segments = packet.segments.map { segment in
+            TranscriptionSegment(id: segment.id,
+                                 text: segment.text,
+                                 language: segment.language,
+                                 firstReceivedTime: _state.transcriptionReceivedTimes[segment.id] ?? Date(),
+                                 lastReceivedTime: Date(),
+                                 isFinal: segment.final)
+        }
+
+        _state.mutate { state in
+            for segment in segments {
+                if segment.isFinal {
+                    state.transcriptionReceivedTimes.removeValue(forKey: segment.id)
+                } else {
+                    state.transcriptionReceivedTimes[segment.id] = segment.firstReceivedTime
+                }
+            }
+        }
+
         delegates.notify {
             $0.room?(self, participant: participant, trackPublication: publication, didReceiveTranscriptionSegments: segments)
         }

Sources/LiveKit/Core/Room.swift

@@ -147,6 +147,9 @@ public class Room: NSObject, ObservableObject, Loggable {
         var publisher: Transport?
         var subscriber: Transport?
         var isSubscriberPrimary: Bool = false
+
+        // Agents
+        var transcriptionReceivedTimes: [String: Date] = [:]
 
         @discardableResult
         mutating func updateRemoteParticipant(info: Livekit_ParticipantInfo, room: Room) -> RemoteParticipant {

Sources/LiveKit/Types/TranscriptionSegment.swift

@@ -21,22 +21,22 @@ public class TranscriptionSegment: NSObject {
     public let id: String
     public let text: String
     public let language: String
-    public let startTime: UInt64
-    public let endTime: UInt64
+    public let firstReceivedTime: Date
+    public let lastReceivedTime: Date
     public let isFinal: Bool
 
     init(id: String,
          text: String,
          language: String,
-         startTime: UInt64,
-         endTime: UInt64,
+         firstReceivedTime: Date,
+         lastReceivedTime: Date,
          isFinal: Bool)
     {
         self.id = id
         self.text = text
         self.language = language
-        self.startTime = startTime
-        self.endTime = endTime
+        self.firstReceivedTime = firstReceivedTime
+        self.lastReceivedTime = lastReceivedTime
         self.isFinal = isFinal
     }
 
@@ -53,16 +53,3 @@ public class TranscriptionSegment: NSObject {
         return hasher.finalize()
     }
 }
-
-// MARK: - Internal
-
-extension Livekit_TranscriptionSegment {
-    func toLKType() -> TranscriptionSegment {
-        TranscriptionSegment(id: id,
-                             text: text,
-                             language: language,
-                             startTime: startTime,
-                             endTime: endTime,
-                             isFinal: final)
-    }
-}