feat(metrics): add metric for builder proxy being up

src/metrics.rs

@@ -1,6 +1,6 @@
 use std::time::Duration;
 
-use metrics::{Counter, Histogram, counter, histogram};
+use metrics::{Counter, Gauge, Histogram, counter, gauge, histogram};
 use metrics_derive::Metrics;
 
 use crate::server::PayloadSource;
@@ -26,6 +26,10 @@ pub struct ServerMetrics {
     #[allow(dead_code)]
     pub builder_rpc_response_count: Counter,
 
+    #[metric(describe = "Whether the builder client is up")]
+    #[allow(dead_code)]
+    pub builder_up: Gauge,
+
     // L2 proxy metrics
     #[metric(describe = "Latency for l2 client forwarded rpc calls (excluding the engine api)", labels = ["method"])]
     #[allow(dead_code)]
@@ -73,6 +77,11 @@ impl ServerMetrics {
         histogram!("rpc.builder_forwarded_call", "method" => method).record(latency.as_secs_f64());
     }
 
+    pub fn record_builder_up(&self, up: bool) {
+        let val = if up { 1 } else { 0 };
+        gauge!("rpc.builder_up").set(val);
+    }
+
     pub fn increment_builder_rpc_response_count(
         &self,
         http_status_code: String,

src/proxy.rs

@@ -339,6 +339,9 @@ async fn record_metrics(
     if let Some(metrics) = &metrics {
         match source {
             PayloadSource::Builder => {
+                metrics.record_builder_up(
+                    http_status_code == StatusCode::INTERNAL_SERVER_ERROR.to_string(),
+                );
                 metrics.record_builder_forwarded_call(duration, method.to_string());
                 metrics.increment_builder_rpc_response_count(
                     http_status_code,