chore(metrics): Comment code to make it easier to understand

src/metrics_utils.rs

@@ -1,12 +1,16 @@
 use metrics::{describe_counter, describe_gauge, describe_histogram};
 
+// counters
 pub const OPENED_TOTAL: &str = "mobc_pool_connections_opened_total";
 pub const CLOSED_TOTAL: &str = "mobc_pool_connections_closed_total";
-pub const OPEN_CONNECTIONS: &str = "mobc_pool_connections_open";
 
+// gauges
+pub const OPEN_CONNECTIONS: &str = "mobc_pool_connections_open";
 pub const ACTIVE_CONNECTIONS: &str = "mobc_pool_connections_busy";
 pub const IDLE_CONNECTIONS: &str = "mobc_pool_connections_idle";
 pub const WAIT_COUNT: &str = "mobc_client_queries_wait";
+
+// histogram
 pub const WAIT_DURATION: &str = "mobc_client_queries_wait_histogram_ms";
 
 pub fn describe_metrics() {