Librados tracing integration

[Ten0]

Plugs librados' logs as tracing events, so that they can be seen by users.

[Xuanwo]

Hi, I assume you're writing this way to minimize the runtime cost of additional checks. However, the enable API itself is usually not a heavy call. How about expanding them into one place within the loop?

for example:

type RadosLogCallback = rados_log_callback_t;

fn librados_log_level() -> (&'static [u8], Option<RadosLogCallback>) {
    const CANDIDATES: &[(Level, &[u8])] = &[
        (Level::DEBUG, b"debug\0"),
        (Level::INFO,  b"info\0"),
        (Level::WARN,  b"warn\0"),
        (Level::ERROR, b"error\0"),
    ];

    for &(lvl, cstr) in CANDIDATES {
        if event_enabled!(target: "librados", lvl) {
            return (cstr, Some(log_callback));
        }
    }

    (b"\0", None)
}

[Ten0]

I think that is not possible because these macros require const level, like below.