Add metrics for RateLimiter

[MartinquaXD]

It would be nice to have some data about how often we need to back off from sending requests because we get rate limited.
I believe the RateLimiter is general pupose enough that we technically could use it for other APIs than Paraswap if the need arises so it would make sense to add labels to the metrics.

Metrics which would make sense:

#[derive(prometheus_metric_storage::MetricStorage, Clone, Debug)]
#[metric(subsystem = "rate_limiter")]
struct Metrics {
    /// Number of requests dropped while backing off.
    requests_dropped: prometheus::CounterVec,
    /// Number of responses indicating a rate limiting error.
    rate_limited_requests: prometheus::CounterVec,
    /// Number of successful requests.
    successful_requests: prometheus::CounterVec,
}