Crate metrics_helper_macros 

Proc-macros for idiomatic Prometheus metrics instrumentation

Provides the #[instrument] attribute macro for ergonomic function instrumentation with counters, histograms, and error tracking.

Quick Start

Metric names are automatically derived from the function name:

use metrics_helper_macros::instrument;

// Auto-generates metrics:
// - counter: "get_users_total"
// - histogram: "get_users_duration_seconds"
// - error_counter: "get_users_errors_total"
#[instrument]
async fn get_users(method: &str) -> Result<Vec<User>, ApiError> {
    // Your code here...
}

You can also override specific metric names:

#[instrument(
    counter = "http_requests_total",  // Override counter name
    labels(endpoint = "/users", method),
)]
async fn get_users(method: &str) -> Result<Vec<User>, ApiError> {
    // Your code here...
}

Labels

Labels can be either static (fixed values) or dynamic (from function parameters):

Static Labels

Use key = "value" syntax for labels with fixed values:

#[instrument(labels(service = "api", version = "v1"))]
fn handle() { }

Dynamic Labels (from function parameters)

Use just the parameter name (without a value) to capture it as a label. The parameter must implement Display:

#[instrument(labels(
    table = "users",  // static: always "users"
    operation,        // dynamic: captured from function param
    tenant_id,        // dynamic: captured from function param
))]
async fn query(operation: &str, tenant_id: &str, limit: usize) -> Result<Data, Error> {
    // Metrics will include: table="users", operation=<value>, tenant_id=<value>
}

This generates metrics like:

query_total{table="users", operation="select", tenant_id="acme-corp"} 1

Dynamic Labels (from struct fields)

Use dot notation (param.field) to capture struct fields as labels. The field must implement Display:

struct Request {
    method: String,
    path: String,
    user_id: u64,
}

#[instrument(labels(
    service = "api",   // static label
    request.method,    // captures request.method as "method" label
    request.path,      // captures request.path as "path" label
))]
fn handle_request(request: &Request) {
    // Metrics will include: service="api", method=<value>, path=<value>
}

You can also specify an explicit key name:

#[instrument(labels(http_method = request.method))]
fn handle(request: &Request) { }

Nested field access is also supported:

#[instrument(labels(ctx.request.method))]
fn process(ctx: &Context) { }

Attribute Macros

instrument

Attribute macro for instrumenting functions with metrics.