Count total requests sent to dex API (#102)

### Context

The oneinch solver on arbitrum currently causes a lot of noisy alerts
which aren't really actionable.
When looking at the alert it appears as if ~40% of the request to the
1inch API return unexpected errors. However, after closer investigation
it seems like this alert actually counts how many of the errors are
unexpected errors.
Specifically for the arbitrum 1inch case <1% of all requests result in
an unexpected error but 40% of all errors are unexpected.

Since 99% of the requests actually result in reasonable responses
(albeit not good enough to satisfy the order's limit price) the 1inch
solver should actually be considered healthy.

### Solution

The error handling logic is surprisingly messy: Errors are grouped in
weird ways, handled in multiple places and used in conjunction with
`Option`.
To resolve the noisy alert ASAP using a minimal patch without having to
rethink all the error handling logic I decided to just introduce another
metric which simply counts all requests sent to the API.
After this gets merged the alert should be updated to alert based on the
ratio `unexpected_errors / total_requests` instead of `unexpected_errors
/ total_errors`

Author: MartinquaXD

src/domain/solver/dex/mod.rs

@@ -155,6 +155,7 @@ impl Dex {
             self.dex
                 .swap(dex_order, &slippage, tokens)
                 .await
+                .inspect(|_| infra::metrics::request_sent())
                 .map_err(dex_err_handler)
         };
         self.rate_limiter

src/infra/metrics.rs

@@ -12,6 +12,9 @@ struct Metrics {
     #[metric(buckets(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15))]
     remaining_time: prometheus::Histogram,
 
+    /// Total number of requests that got sent to the DEX API.
+    solve_requests: prometheus::IntCounter,
+
     /// Errors that occurred during solving.
     #[metric(labels("reason"))]
     solve_errors: prometheus::IntCounterVec,
@@ -46,6 +49,10 @@ pub fn solve_error(reason: &str) {
     get().solve_errors.with_label_values(&[reason]).inc();
 }
 
+pub fn request_sent() {
+    get().solve_requests.inc();
+}
+
 /// Get the metrics instance.
 fn get() -> &'static Metrics {
     Metrics::instance(observe::metrics::get_storage_registry())