Show the resolved default settings path in --help; add Prometheus/OpenMetrics polling

--settings-path's help text now names the actual resolved default path
(computed once via LazyLock) instead of only describing where to look for
one. Also adds an optional --metrics-url flag that polls a Prometheus/
OpenMetrics-compatible endpoint on an interval, pushing samples straight
into a metric's bucket (bypassing log-line regex matching) so they're
queryable via PromQL/dashboards alongside log-derived metrics.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/engine.rs

@@ -23,6 +23,15 @@ pub struct IngestedLine {
     pub text: String,
 }
 
+/// One sample scraped from a Prometheus/OpenMetrics endpoint, pushed straight into a metric's bucket
+/// without going through log-line regex matching — see `crate::ingest::run_metrics_poll`.
+pub struct MetricSample {
+    pub metric_name: String,
+    pub labels: BTreeMap<String, String>,
+    pub value: f64,
+    pub timestamp: Timestamp,
+}
+
 /// Also the request body for `POST /api/params` — `Deserialize` is derived directly here rather than adding a separate DTO, since the shape already matches.
 #[derive(Debug, Clone, Deserialize)]
 pub struct DeclareParamRequest {
@@ -69,6 +78,7 @@ pub enum EngineQuery {
 #[allow(dead_code)]
 pub enum EngineMessage {
     Ingest(IngestedLine),
+    IngestMetricSample(MetricSample),
     DeclareParam {
         request: DeclareParamRequest,
         respond_to: oneshot::Sender<Result<(), String>>,
@@ -114,6 +124,8 @@ pub async fn run(mut rx: mpsc::Receiver<EngineMessage>, settings: SettingsHandle
     let mut index: BTreeMap<IndexKey, DataValue> = BTreeMap::new();
     let mut max_logs: Option<usize> = None;
     let mut log_count: usize = 0;
+    // Its own counter, independent of log `sequence` — scraped samples aren't tied to any ingested line.
+    let mut metric_sample_sequence: u64 = 0;
     // Per-rule rolling windows/seen-value sets, keyed by rule id; lives only as long as this loop (no restart persistence).
     let mut alert_states: HashMap<String, AlertRuntimeState> = HashMap::new();
 
@@ -139,6 +151,22 @@ pub async fn run(mut rx: mpsc::Receiver<EngineMessage>, settings: SettingsHandle
                     tokio::spawn(alerts::deliver_for_rule(rule.name, message, ntfy_topic.clone(), ntfy_server.clone()));
                 }
             }
+            EngineMessage::IngestMetricSample(sample) => {
+                let labels = sample
+                    .labels
+                    .into_iter()
+                    .map(|(name, value)| (name, MetricPrimitive::String(value)))
+                    .collect();
+                push_bucket_value(
+                    &mut index,
+                    &sample.metric_name,
+                    labels,
+                    metric_sample_sequence,
+                    sample.timestamp,
+                    MetricPrimitive::Number(Number::Float(sample.value)),
+                );
+                metric_sample_sequence += 1;
+            }
             EngineMessage::SetMaxLogs(new_max) => {
                 max_logs = new_max;
                 evict_excess_logs(&mut index, &mut log_count, max_logs);