Function insert_row 

fn insert_row(
    snapshot: &mut BTreeMap<(String, Vec<(String, String)>), (f64, &'static str, String)>,
    name: String,
    labels: Vec<(String, String)>,
    value: f64,
    metric_type: &'static str,
    help: &str,
)

Insert a single metric row into the snapshot.