Add quilkin_allocated_xdp_packets metric (#1357)

This tracks the number of packets that are currently "allocated" from
the Umem, either waiting to be filled with received data, or waiting to
be sent

This tracks a single metric, as I don't think we can track it on a per
queue basis due to cardinality, but I might be wrong about that

Resolves: #1168

Author: Jake-Shadle

Cargo.lock

@@ -640,6 +640,21 @@ pub(crate) fn provider_task_failures_total(provider_task: &str) -> IntCounter {
     PROVIDER_TASK_FAILURES_TOTAL.with_label_values(&[provider_task])
 }
 
+pub(crate) fn allocated_xdp_packets() -> &'static IntGauge {
+    static ALLOCATED: Lazy<IntGauge> = Lazy::new(|| {
+        prometheus::register_int_gauge_with_registry! {
+            prometheus::opts! {
+                "quilkin_allocated_xdp_packets",
+                "The number of packets that are allocated from a UMEM",
+            },
+            registry(),
+        }
+        .unwrap()
+    });
+
+    &ALLOCATED
+}
+
 /// Create a generic metrics options.
 /// Use `filter_opts` instead if the intended target is a filter.
 pub fn opts(name: &str, subsystem: &str, description: &str) -> Opts {

src/metrics.rs

@@ -429,6 +429,9 @@ fn io_loop(
     let mut rx_slab = xdp::slab::StackSlab::<BATCH_SIZE>::new();
     let mut tx_slab = xdp::slab::StackSlab::<{ BATCH_SIZE << 2 }>::new();
     let mut pending_sends = 0;
+    let mut outstanding = umem.outstanding() as i64;
+
+    crate::metrics::allocated_xdp_packets().add(outstanding);
 
     // SAFETY: the cases of unsafe in this code block all concern the relationship
     // between frames and the Umem, the frames cannot outlive the Umem which is
@@ -492,6 +495,10 @@ fn io_loop(
             // Return frames that have completed sending
             pending_sends += enqueued_sends;
             pending_sends -= completion.dequeue(&mut umem, pending_sends);
+
+            let new = umem.outstanding() as i64;
+            crate::metrics::allocated_xdp_packets().add(new - outstanding);
+            outstanding = new;
         }
     }
 }