throttle backend metrics to once every 5 seconds (#841)

Instead of passing through backend metrics every second - let's just
take one metrics event every 5 seconds.
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----

> [!IMPORTANT]
> Throttle backend metrics processing in `metrics_loop` to once every 5
seconds using `METRICS_INTERVAL`.
> 
>   - **Behavior**:
> - Throttle backend metrics processing in `metrics_loop` in
`metrics.rs` to once every 5 seconds using `METRICS_INTERVAL`.
>     - Metrics are skipped if processed within the last 5 seconds.
>   - **Constants**:
> - Add `METRICS_INTERVAL` constant set to 5 seconds in `metrics.rs`.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=jamsocket%2Fplane&utm_source=github&utm_medium=referral)<sup>
for 3d25cc1. It will automatically
update as commits are pushed.</sup>

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plane/src/drone/runtime/docker/metrics.rs

@@ -3,8 +3,11 @@ use crate::{database::backend::BackendMetricsMessage, names::BackendName};
 use bollard::{container::StatsOptions, Docker};
 use futures_util::Stream;
 use std::sync::{Arc, Mutex};
+use tokio::time::{Duration, Instant};
 use tokio_stream::StreamExt;
 
+const METRICS_INTERVAL: Duration = Duration::from_secs(5);
+
 fn stream_metrics(
     docker: &Docker,
     container_id: &ContainerId,
@@ -23,10 +26,17 @@ pub async fn metrics_loop(
     callback: Arc<Mutex<Option<MetricsCallback>>>,
 ) {
     let container_id = ContainerId::from(&backend_id);
-
     let mut stream = stream_metrics(&docker, &container_id);
+    let mut last_processed = Instant::now() - METRICS_INTERVAL;
 
     while let Some(stats) = stream.next().await {
+        // These metrics come from Docker every second, so let's throttle them to be less frequent.
+        let now = Instant::now();
+        if now.duration_since(last_processed) < METRICS_INTERVAL {
+            continue;
+        }
+        last_processed = now;
+
         let stats = match stats {
             Err(err) => {
                 tracing::error!(?err, "Error getting metrics for {container_id}");