Break apart metrics into their own module (#335)

Signed-off-by: José Ulises Niño Rivera <junr03@users.noreply.github.com>

crates/prompt_gateway/src/metrics.rs

@@ -0,0 +1,14 @@
+use common::stats::Gauge;
+
+#[derive(Copy, Clone, Debug)]
+pub struct Metrics {
+    pub active_http_calls: Gauge,
+}
+
+impl Metrics {
+    pub fn new() -> Metrics {
+        Metrics {
+            active_http_calls: Gauge::new(String::from("active_http_calls")),
+        }
+    }
+}