docs: Add metrics overview and examples to metrics.md

docs/src/fancyanalytics/features/metrics.md

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+# Metrics
+
+Metrics are **numerical values** that represent the state, performance, or behavior of your application over time. 
+They are the core building blocks of the analytics platform — enabling insights, dashboards, alerts, and aggregation across many data sources.
+
+## What is a Metric?
+
+A **metric** is:
+
+- Identified by a **name** (e.g. `cpu_usage_percent`, `online_players`, `npc_type_pig`)
+- Reported by one or more **senders** (e.g., a game server, plugin, or application)
+- Includes exact one **numeric value**
+
+Each metric is sent at a point in time and represents a measurable value from your application or environment.
+
+## What Can Be a Metric?
+
+Anything that can be measured numerically:
+
+| Metric Name         | Example Value | Description                         |
+|---------------------|---------------|-------------------------------------|
+| `cpu_usage_percent` | `42.5`        | Current CPU usage in percent        |
+| `online_players`    | `10`          | Number of players online            |
+| `npc_count`         | `123`         | Total NPCs spawned on a game server |
+| `npc_count_pig`     | `5`           | Number of pig NPCs                  |
+| `plugin_enabled`    | `1` (true)    | Boolean values are converted to 1/0 |
+| `tps`               | `20`          | Ticks per second                    |
+| `network_in_kb`     | `512`         | Incoming network traffic            |
+