"""OpenTelemetry setup with Prometheus metric export."""

from opentelemetry import metrics
from opentelemetry.exporter.prometheus import PrometheusMetricReader
from opentelemetry.sdk.metrics import MeterProvider
from prometheus_client import start_http_server


def setup_telemetry(service_name: str, metrics_port: int = 9090) -> metrics.Meter:
    """Initialise an OpenTelemetry MeterProvider backed by Prometheus.

    A Prometheus-compatible HTTP server is started on *metrics_port*
    so that an external Prometheus instance can scrape ``/metrics``.

    Returns a ``Meter`` that services use to create counters, histograms, etc.
    """
    reader = PrometheusMetricReader()
    provider = MeterProvider(metric_readers=[reader])
    metrics.set_meter_provider(provider)
    start_http_server(metrics_port)
    return metrics.get_meter(service_name)