Overhaul stats: Refactor HTTP metrics

[josecelano]

There are a couple of comments like this:

/// > **NOTICE**: as the HTTP tracker does not requires a connection request
/// > like the UDP tracker, the number of TCP connections is incremented for
/// > each `announce` request.

The HTTP tracker core keeps a counter for the number of connections handled. However that metrics does not make sense. Internally the counter is the number of announce requests plus the number of scrape requests.

pub struct Metrics {
    /// Total number of TCP (HTTP tracker) connections from IPv4 peers.
    /// Since the HTTP tracker spec does not require a handshake, this metric
    /// increases for every HTTP request.
    pub tcp4_connections_handled: u64,

    /// Total number of TCP (HTTP tracker) `announce` requests from IPv4 peers.
    pub tcp4_announces_handled: u64,

    /// Total number of TCP (HTTP tracker) `scrape` requests from IPv4 peers.
    pub tcp4_scrapes_handled: u64,

    /// Total number of TCP (HTTP tracker) connections from IPv6 peers.
    pub tcp6_connections_handled: u64,

    /// Total number of TCP (HTTP tracker) `announce` requests from IPv6 peers.
    pub tcp6_announces_handled: u64,

    /// Total number of TCP (HTTP tracker) `scrape` requests from IPv6 peers.
    pub tcp6_scrapes_handled: u64,
}

I think it would make sense to delete those counters and add new counter to the axum-http-tracker-server package:

• tcp4_requests
• tcp6_requests

The udp-tracker-server has those metrics and also:

• tcp4_responses
• tcp6_responses