Implement KLV datachanel for WebRTC

[onorua]

This implementation adds comprehensive KLV (Key-Length-Value) metadata support to MediaMTX's WebRTC functionality. KLV metadata is transmitted via WebRTC data channels, allowing real-time delivery of telemetry and metadata alongside video and audio streams.

Implementation Details

Data Channel Configuration

dataChannelInit := &webrtc.DataChannelInit{
    Ordered:        &[]bool{true}[0],  // Ensure ordered delivery
    MaxRetransmits: &[]uint16{3}[0],   // Allow some retransmissions
}

KLV Format Detection Logic

// Check for Generic format with KLV RTPMap
if genericFmt, ok := forma.(*format.Generic); ok {
    if genericFmt.RTPMap() == "KLV/90000" {
        klvFormat = genericFmt
        klvMedia = media
        break
    }
}

// Also check for internal KLV format
if forma.Codec() == "KLV" {
    klvFormat = forma
    klvMedia = media
    break
}

JavaScript KLV Data Handling

dataChannel.onmessage = (event) => {
    if (this.conf.onKLVData !== undefined) {
        const klvData = new Uint8Array(event.data);
        this.conf.onKLVData(klvData);
    }
};

Usage

Basic WebRTC with KLV

new MediaMTXWebRTCReader({
    url: new URL('whep', window.location.href),
    onTrack: (evt) => {
        video.srcObject = evt.streams[0];
    },
    onKLVData: (klvData) => {
        console.log('KLV data received:', klvData);
        // Process KLV metadata
    }
});

[codecov]

Codecov Report

Attention: Patch coverage is 40.65041% with 73 lines in your changes missing coverage. Please review.

Project coverage is 54.55%. Comparing base (97d2fff) to head (7808bce).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/protocols/webrtc/from_stream.go	37.17%	41 Missing and 8 partials ⚠️
internal/protocols/webrtc/peer_connection.go	46.66%	20 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4722      +/-   ##
==========================================
- Coverage   54.61%   54.55%   -0.06%     
==========================================
  Files         209      210       +1     
  Lines       21785    21913     +128     
==========================================
+ Hits        11897    11955      +58     
- Misses       8978     9037      +59     
- Partials      910      921      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[llamafilm]

How is this meant to be used? I don't see anything in mediamtx.yml about how to provide this data.
Edit: never mind! Looking at older issues I see the KLV data comes from the source stream.