# Metric.msg

## Overview

`Metric.msg` represents a single metric value, optionally labeled, that can be one of three types: counter, gauge, or histogram. It mirrors the metric model used internally by Cartographer's metrics infrastructure.

## Constants

| Constant | Value | Meaning |
| --- | --- | --- |
| `TYPE_COUNTER` | `0` | A monotonically increasing counter. |
| `TYPE_GAUGE` | `1` | A value that can increase and decrease. |
| `TYPE_HISTOGRAM` | `2` | A distribution of values across buckets. |

## Fields

| Field | Type | Description |
| --- | --- | --- |
| `type` | `uint8` | One of the `TYPE_*` constants above. |
| `labels` | `cartographer_ros_msgs/MetricLabel[]` | Key/value labels attached to this metric. |
| `value` | `float64` | The metric value, valid for `TYPE_COUNTER` or `TYPE_GAUGE`. |
| `counts_by_bucket` | `cartographer_ros_msgs/HistogramBucket[]` | The per-bucket counts, valid for `TYPE_HISTOGRAM`. |

## Usage

- For counters and gauges, populate `value` and leave `counts_by_bucket` empty.
- For histograms, populate `counts_by_bucket`; `value` is unused.
- Metrics are grouped into families via `MetricFamily.msg`.