Macro flow_begin 

macro_rules! flow_begin {
    ($category:tt, $label:tt, $flow_id:expr $(, $key:tt => $val:expr)* $(,)?) => { ... };
}

Writes a flow begin event associated with the current thread when the given category is enabled.

Arguments:

• category: Filter category for the event. Expects a string literal or expression.
• label: Label for the event. Expects a string literal or expression.
• flow_id: Flow id for the event. Must be convertible to u64.
• …: List of key => value argument pairs.