Cost tracking

pub struct ModelPrice {
    pub input: f64,        // USD per 1M input tokens
    pub output: f64,
    pub cache_read: f64,
    pub cache_write: f64,
}

use cognis_trace::TracingHandler;

let handler = TracingHandler::builder()
    .with_exporter(my_exporter)
    .with_default_pricing()
    .build();

use cognis_trace::{cost::ModelPrice, TracingHandler};

let handler = TracingHandler::builder()
    .with_exporter(my_exporter)
    .with_default_pricing()
    .override_price("gpt-4o", ModelPrice {
        input: 2.50,
        output: 10.00,
        cache_read: 1.25,
        cache_write: 0.0,
    })
    .build();

use cognis_trace::{PriceTable, TracingHandler};

let mut prices = PriceTable::default();
prices.insert("my-model", ModelPrice { /* … */ });
prices.insert("my-model-mini", ModelPrice { /* … */ });

let handler = TracingHandler::builder()
    .with_exporter(my_exporter)
    .with_pricing(prices)
    .build();

use cognis::prelude::*;

struct CostBudget { limit_usd: f64, /* … */ }

impl Observer for CostBudget {
    fn on_event(&self, e: &Event) {
        if let Event::OnEnd { output, .. } = e {
            // The output JSON contains usage and computed cost when emitted by an LLM Runnable.
            // Inspect and decide.
        }
    }
}