Feat/expose health and raft metrics

[andriimredocly]

I want to expose the health and the raft state metrics
Two new gauges are added:
typesense_health: 1 if the target responds with 200 HTTP code and the response is {"ok": true}

# HELP typesense_health Health status of Typesense instance (1 = healthy, 0 = unhealthy)
# TYPE typesense_health gauge

and typesense_raft_state: 1 = leader, 4 = follower, 0 = other

# HELP typesense_raft_state Raft state of Typesense node (1 = leader, 4 = follower, 0 = other)
# TYPE typesense_raft_state gauge

This is useful as I am running Typesense in HA mode with 3 instances, and I need to configure alerts for a missing leader.

Also, I see that the metrics exporter defaults metric if the 401 response is sent:

Define default:

    let mut stats_data: TypesenseStats = TypesenseStats::default();

Update if the HTTP code is 200

        reqwest::StatusCode::OK => {
            match res.json::<TypesenseStats>().await {
                Ok(parsed) => {
                    stats_data = parsed;
                }
                Err(_) => println!("Hm, the response didn't match the shape we expected."),
            };
        }

Return

    Ok(stats_data)

But if the response fails completely or times out, nothing happens
I added the timeout for the request that is disabled by default and added the error handler for failed requests and timeouts

    let res = match client
        .get(url)
        .header("X-TYPESENSE-API-KEY", format!("{}", args.typesense_api_key))
        .send()
        .await
    {
        Ok(res) => res,
        Err(e) => {
            println!("Stats endpoint: request failed (timeout/connection error): {:?}", e);
            return Ok(stats_data);
        }
    };

This is also related to this issue: #2