Implement support for laminar metrics.

[TimonPost]

Task Description

Laminar currently does not provide much internal information to the outside. To solve this we need to have metrics, and a way to integrate this easily.

Task TODO

• Think of metrics we can collect
• Are there crates that can help us with this
• Think of design to move metrics into our current API without too many changes.

[TimonPost]

My idea to implement this is the following:

1. We can make SocketEvent have an entry for Metrics:

pub enum SocketEvent {
    /// A packet has been received from a client.
    Packet(Packet),
    /// A new client connects. Clients are uniquely identified by the ip:port combination at this layer.
    Connect(SocketAddr),
    /// A client disconnects. This is generated from the server-side intentionally disconnecting a client,
    /// or it could be from the client disconnecting.
    Disconnect(SocketAddr),
    /// This is generated if the server has not seen traffic from a client after a configurable amount of time.
    Timeout(SocketAddr),
    
    /// Metrics of lamianar
    Metrics(Metrics)
}

2. Example of metrics we can gather:
   source

pub struct Metrics {
    counters: [u64; DataPoint::Length as usize],
    packet_loss: f32,
    sent_bandwidth_kbps: f32,
    received_bandwidth_kbps: f32,
    acked_bandwidth_kbps: f32,

    // Config values to tweak the calculated fields
    bandwidth_smoothing_factor: f32,
}

3. Send metrics back to the user
   Because we created a Metrics entry for the SocketEvent we can send it on the EventChannel

4. Add a config entry

• Metrics Enabled; whether we should calculate metrics or not.
• Metrics send interval; send on event channel.

[ncallaway]

It seems like this could potentially be combined with:

• Expose active connections (Access to Socket's ActiveConnections? #271)
• Expose RTT information (Expose RTT information. #268)

I might try and roll all of those into a single interface and PR.

Should metrics be on an event, or on an on-demand poll?

[TimonPost]

Thats a good question. I tended to see them as an event like my comment above shows.