Method histogram

src/adapter/src/client.rs:1623–1650

(
        client: &SessionClient,
        instance_id: Option<ComputeInstanceId>,
        strategy: Option<StatementExecutionStrategy>,
    )

    }

    fn histogram(
        client: &SessionClient,
        instance_id: Option<ComputeInstanceId>,
        strategy: Option<StatementExecutionStrategy>,
    ) -> Histogram {
        let session = client.session.as_ref().expect("session invariant");
        let isolation_level = *session.vars().transaction_isolation();
        let name_hint = ApplicationNameHint::from_str(session.application_name());
        let instance = match instance_id {
            Some(i) => Cow::Owned(i.to_string()),
            None => Cow::Borrowed("none"),
        };
        let strategy = match strategy {
            Some(s) => s.name(),
            None => "none",
        };

        client
            .inner()
            .metrics()
            .time_to_first_row_seconds
            .with_label_values(&[
                instance.as_ref(),
                isolation_level.as_variant_str(),
                strategy,
                name_hint.as_str(),
            ])
    }

    /// If you want to match [`RecordFirstRowStream`]'s logic but don't need
    /// a UnboundedReceiver, you can tell it when to record an observation.