Prometheus encoding (#1525)

Continuation of #1068

Addressed some of the concerns in the reviews:
- Move `PrometheusMetric` method to original type where possible
- Move `writeResponse` logic to api package
- Improve handling slices of Prometheus marshallable types
- Disable type checking in `jape`

.github/workflows/test.yml

@@ -25,6 +25,8 @@ jobs:
             autopilot
             bus bus/client
             worker worker/client
+          flags: |
+            -japecheck.types=false
   test:
     needs: analyze
     runs-on: ${{ matrix.os }}

alerts/prometheus.go

@@ -0,0 +1,24 @@
+package alerts
+
+import "go.sia.tech/renterd/internal/prometheus"
+
+// PrometheusMetric implements prometheus.Marshaller.
+func (a Alert) PrometheusMetric() (metrics []prometheus.Metric) {
+	metrics = append(metrics, prometheus.Metric{
+		Name: "renterd_alert",
+		Labels: map[string]any{
+			"id":        a.ID,
+			"severity":  a.Severity.String(),
+			"message":   a.Message,
+			"timestamp": a.Timestamp,
+		},
+		Value: 1,
+	})
+	return
+}
+
+// PrometheusMetric implements prometheus.Marshaller.
+func (a AlertsResponse) PrometheusMetric() (metrics []prometheus.Metric) {
+	metrics = prometheus.Slice(a.Alerts).PrometheusMetric()
+	return
+}