# Syntax
```text
(control_param),*
```
where
- `control_param`: A Rust field-value with a pre-determined identifier (see below).
# Control parameters
This macro accepts the following optional control parameters:
| `rt` | `impl emit::runtime::Runtime` | The runtime to emit the metric sample through if `sampler` is unspecified. |
| `sampler` | `impl emit::sampler::Sampler` | The `Sampler` to pass the metric sample to. |
| `mdl` | `impl Into<emit::Path>` | The module the metric sample belongs to. If unspecified the current module path is used. |
| `extent` | `impl emit::ToExtent` | The extent to use on the metric sample. If unspecified the extent is left empty. |
| `props` | `impl emit::Props` | A set of properties to add to the metric sample. |
| `value` (**required**) | `impl emit::ToValue` | The value of the metric sample. If the value is an identifier then `name` will be inferred to be that identifier. |
| `name` (**required** if `value` is not an identifier) | `impl emit::ToStr` | The name of the metric being sampled. If unspecified, and `value` is an identifier, then the stringified identifier is used as the name. |
| `agg` | `impl emit::ToStr` | The aggregation of the metric sample. If unspecified, the default for the macro is used. |