deployment/alerts: add RemoteWriteDroppingData to vmalert rules (#7393)

### Describe Your Changes

Please provide a brief description of the changes you made. Be as
specific as possible to help others understand the purpose and impact of
your modifications.

### Checklist

The following checks are **mandatory**:

- [ ] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>

deployment/docker/rules/alerts-vmalert.yml

@@ -74,6 +74,17 @@ groups:
           description: "vmalert instance {{ $labels.instance }} is failing to push metrics generated via alerting 
             or recording rules to the configured remote write URL. Check vmalert's logs for detailed error message."
 
+      - alert: RemoteWriteDroppingData
+        expr: increase(vmalert_remotewrite_dropped_rows_total[5m]) > 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "vmalert instance {{ $labels.instance }} is dropping data sent to remote write URL"
+          description: "vmalert instance {{ $labels.instance }} is failing to send results of alerting or recording rules 
+            to the configured remote write URL. This may result into gaps in recording rules or alerts state.
+            Check vmalert's logs for detailed error message."
+
       - alert: AlertmanagerErrors
         expr: increase(vmalert_alerts_send_errors_total[5m]) > 0
         for: 15m

docs/changelog/CHANGELOG.md

@@ -23,6 +23,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/): support scraping from Kubernetes Native Sidecars. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7287).
 * FEATURE: [Single-node VictoriaMetrics](https://docs.victoriametrics.com/) and `vmstorage` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/cluster-victoriametrics/): add a separate cache type for storing sparse entries when performing large index scans. This significantly reduces memory usage when applying [downsampling filters](https://docs.victoriametrics.com/#downsampling) and [retention filters](https://docs.victoriametrics.com/#retention-filters) during background merge. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7182) for the details.
 * FEATURE: [dashboards](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/dashboards) for VM single-node, cluster, vmalert, vmagent, VictoriaLogs: add `Restarts` panel to show the events of process restarts. This panel should help correlate events of restart with unexpected behavior of processes.
+* FEATURE: [alerts](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/deployment/docker/rules/alerts-vmalert.yml): add alerting rule `RemoteWriteDroppingData` to track number of dropped samples that weren't sent to remote write URL.
 
 * BUGFIX: [dashboards](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/dashboards) for Single-node VictoriaMetrics, cluster: The free disk space calculation now will subtract the size of the `-storage.minFreeDiskSpaceBytes` flag to correctly display the remaining available space of Single-node VictoriaMetrics/vmstorage rather than the actual available disk space, as well as the full ETA. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7334) for the details.
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert): properly set `group_name` and `file` fields for recording rules in `/api/v1/rules`.