//! The meter provider a swarm service pushes its OTLP metrics through.
//!
//! Built from the standard `OTEL_*` environment: the endpoint from
//! `OTEL_EXPORTER_OTLP_ENDPOINT` and the resource from
//! `OTEL_RESOURCE_ATTRIBUTES`. With no endpoint a service exports nothing, so
//! a run outside the swarm (a test, a dev shell) needs no collector.

use std::time::Duration;

use opentelemetry_otlp::{MetricExporter, Protocol, WithExportConfig};
use opentelemetry_sdk::metrics::{PeriodicReader, SdkMeterProvider};

/// How often the metrics are exported.
const INTERVAL: Duration = Duration::from_mins(1);

/// The provider for the service `cert` names in the `bao.client_cert.*` gauges
/// it also registers on a meter called `meter`, or `None` when no collector is
/// configured or the exporter failed to build.
///
/// The caller keeps the provider alive: it exports only while it lives.
#[must_use]
pub fn from_env(meter: &'static str, cert: &'static str) -> Option<SdkMeterProvider> {
    use opentelemetry::metrics::MeterProvider as _;

    if !std::env::var("OTEL_EXPORTER_OTLP_ENDPOINT").is_ok_and(|s| !s.trim().is_empty()) {
        return None;
    }
    // The blocking client: `PeriodicReader` exports from a thread of its own
    // with no Tokio reactor.
    let exporter = match MetricExporter::builder()
        .with_http()
        .with_protocol(Protocol::HttpJson)
        .build()
    {
        Ok(exporter) => exporter,
        Err(e) => {
            tracing::warn!(error = %e, meter, "building the metric exporter failed; no metrics are exported");
            return None;
        }
    };
    let reader = PeriodicReader::builder(exporter)
        .with_interval(INTERVAL)
        .build();
    let provider = SdkMeterProvider::builder().with_reader(reader).build();
    crate::cert_metrics::register(&provider.meter(meter), cert);
    Some(provider)
}