chore: rename metrics skill to observability, add logs reference

Merge Prometheus metrics and Loki logs into a unified troubleshooting
skill. Adds LogQL query patterns, label reference, and common service
units for log searching.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/skills/observability/SKILL.md

@@ -0,0 +1,250 @@
+---
+name: observability
+description: Reference guide for exploring Prometheus metrics and Loki logs when troubleshooting homelab issues. Use when investigating system state, deployments, service health, or searching logs.
+---
+
+# Observability Troubleshooting Guide
+
+Quick reference for exploring Prometheus metrics and Loki logs to troubleshoot homelab issues.
+
+## Available Tools
+
+Use the `lab-monitoring` MCP server tools:
+
+**Metrics:**
+- `search_metrics` - Find metrics by name substring
+- `get_metric_metadata` - Get type/help for a specific metric
+- `query` - Execute PromQL queries
+- `list_targets` - Check scrape target health
+- `list_alerts` / `get_alert` - View active alerts
+
+**Logs:**
+- `query_logs` - Execute LogQL queries against Loki
+- `list_labels` - List available log labels
+- `list_label_values` - List values for a specific label
+
+---
+
+## Logs Reference
+
+### Label Reference
+
+Available labels for log queries:
+- `host` - Hostname (e.g., `ns1`, `monitoring01`, `ha1`)
+- `systemd_unit` - Systemd unit name (e.g., `nsd.service`, `nixos-upgrade.service`)
+- `job` - Either `systemd-journal` (most logs) or `varlog` (file-based logs)
+- `filename` - For `varlog` job, the log file path
+- `hostname` - Alternative to `host` for some streams
+
+### Log Format
+
+Journal logs are JSON-formatted. Key fields:
+- `MESSAGE` - The actual log message
+- `PRIORITY` - Syslog priority (6=info, 4=warning, 3=error)
+- `SYSLOG_IDENTIFIER` - Program name
+
+### Basic LogQL Queries
+
+**Logs from a specific service on a host:**
+```logql
+{host="ns1", systemd_unit="nsd.service"}
+```
+
+**All logs from a host:**
+```logql
+{host="monitoring01"}
+```
+
+**Logs from a service across all hosts:**
+```logql
+{systemd_unit="nixos-upgrade.service"}
+```
+
+**Substring matching (case-sensitive):**
+```logql
+{host="ha1"} |= "error"
+```
+
+**Exclude pattern:**
+```logql
+{host="ns1"} != "routine"
+```
+
+**Regex matching:**
+```logql
+{systemd_unit="prometheus.service"} |~ "scrape.*failed"
+```
+
+**File-based logs (caddy access logs, etc):**
+```logql
+{job="varlog", hostname="nix-cache01"}
+{job="varlog", filename="/var/log/caddy/nix-cache.log"}
+```
+
+### Time Ranges
+
+Default lookback is 1 hour. Use `start` parameter for older logs:
+- `start: "1h"` - Last hour (default)
+- `start: "24h"` - Last 24 hours
+- `start: "168h"` - Last 7 days
+
+### Common Services
+
+Useful systemd units for troubleshooting:
+- `nixos-upgrade.service` - Daily auto-upgrade logs
+- `nsd.service` - DNS server (ns1/ns2)
+- `prometheus.service` - Metrics collection
+- `loki.service` - Log aggregation
+- `caddy.service` - Reverse proxy
+- `home-assistant.service` - Home automation
+- `step-ca.service` - Internal CA
+- `openbao.service` - Secrets management
+- `sshd.service` - SSH daemon
+- `nix-gc.service` - Nix garbage collection
+
+### Extracting JSON Fields
+
+Parse JSON and filter on fields:
+```logql
+{systemd_unit="prometheus.service"} | json | PRIORITY="3"
+```
+
+---
+
+## Metrics Reference
+
+### Deployment & Version Status
+
+Check which NixOS revision hosts are running:
+
+```promql
+nixos_flake_info
+```
+
+Labels:
+- `current_rev` - Git commit of the running NixOS configuration
+- `remote_rev` - Latest commit on the remote repository
+- `nixpkgs_rev` - Nixpkgs revision used to build the system
+- `nixos_version` - Full NixOS version string (e.g., `25.11.20260203.e576e3c`)
+
+Check if hosts are behind on updates:
+
+```promql
+nixos_flake_revision_behind == 1
+```
+
+View flake input versions:
+
+```promql
+nixos_flake_input_info
+```
+
+Labels: `input` (name), `rev` (revision), `type` (git/github)
+
+Check flake input age:
+
+```promql
+nixos_flake_input_age_seconds / 86400
+```
+
+Returns age in days for each flake input.
+
+### System Health
+
+Basic host availability:
+
+```promql
+up{job="node-exporter"}
+```
+
+CPU usage by host:
+
+```promql
+100 - (avg by (instance) (rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100)
+```
+
+Memory usage:
+
+```promql
+1 - (node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes)
+```
+
+Disk space (root filesystem):
+
+```promql
+node_filesystem_avail_bytes{mountpoint="/"} / node_filesystem_size_bytes{mountpoint="/"}
+```
+
+### Service-Specific Metrics
+
+Common job names:
+- `node-exporter` - System metrics (all hosts)
+- `nixos-exporter` - NixOS version/generation metrics
+- `caddy` - Reverse proxy metrics
+- `prometheus` / `loki` / `grafana` - Monitoring stack
+- `home-assistant` - Home automation
+- `step-ca` - Internal CA
+
+### Instance Label Format
+
+The `instance` label uses FQDN format:
+
+```
+<hostname>.home.2rjus.net:<port>
+```
+
+Example queries filtering by host:
+
+```promql
+up{instance=~"monitoring01.*"}
+node_load1{instance=~"ns1.*"}
+```
+
+---
+
+## Troubleshooting Workflows
+
+### Check Deployment Status Across Fleet
+
+1. Query `nixos_flake_info` to see all hosts' current revisions
+2. Check `nixos_flake_revision_behind` for hosts needing updates
+3. Look at upgrade logs: `{systemd_unit="nixos-upgrade.service"}` with `start: "24h"`
+
+### Investigate Service Issues
+
+1. Check `up{job="<service>"}` for scrape failures
+2. Use `list_targets` to see target health details
+3. Query service logs: `{host="<host>", systemd_unit="<service>.service"}`
+4. Search for errors: `{host="<host>"} |= "error"`
+5. Check `list_alerts` for related alerts
+
+### After Deploying Changes
+
+1. Verify `current_rev` updated in `nixos_flake_info`
+2. Confirm `nixos_flake_revision_behind == 0`
+3. Check service logs for startup issues
+4. Check service metrics are being scraped
+
+### Debug SSH/Access Issues
+
+```logql
+{host="<host>", systemd_unit="sshd.service"}
+```
+
+### Check Recent Upgrades
+
+```logql
+{systemd_unit="nixos-upgrade.service"}
+```
+
+With `start: "24h"` to see last 24 hours of upgrades across all hosts.
+
+---
+
+## Notes
+
+- Default scrape interval is 15s for most metrics targets
+- Default log lookback is 1h - use `start` parameter for older logs
+- Use `rate()` for counter metrics, direct queries for gauges
+- The `instance` label includes the port, use regex matching (`=~`) for hostname-only filters
+- Log `MESSAGE` field contains the actual log content in JSON format