pelikan-net: make metrics opt-in

[brayniac]

Changes the way metrics are included in pelikan-net, moving them behind a feature flag to make them opt-in.

[swlynch99]

@thinkingfish pointed me to this PR. Here's a suggestion that lets you cut down on needing to have #[cfg(feature = "metrics")] everywhere. You can define a metric! macro that conditionally expands to either nothing or its input:

#[cfg(feature = "metrics")]
macro_rules! metrics {
    { $( $tt:tt )* } => { $( $tt )* }
}

#[cfg(not(feature = "metrics"))]
macro_rules! metrics {
    { $( $tt:tt)* } => {}
}

Then, later on, instead of using #[cfg(feature = "metrics")] you wrap the metrics-related code in a metrics! macro

metrics! {
    METRIC.increment();
}

You still have to modify all the calls but you don't have to repeat #[cfg] everywhere so it is easier to change in the future if you need to.

[brayniac]

@thinkingfish pointed me to this PR. Here's a suggestion that lets you cut down on needing to have #[cfg(feature = "metrics")] everywhere. You can define a metric! macro that conditionally expands to either nothing or its input:

#[cfg(feature = "metrics")]
macro_rules! metrics {
    { $( $tt:tt )* } => { $( $tt )* }
}

#[cfg(not(feature = "metrics"))]
macro_rules! metrics {
    { $( $tt:tt)* } => {}
}

Then, later on, instead of using #[cfg(feature = "metrics")] you wrap the metrics-related code in a metrics! macro

metrics! {
    METRIC.increment();
}

You still have to modify all the calls but you don't have to repeat #[cfg] everywhere so it is easier to change in the future if you need to.

Not a bad idea!

[swlynch99]

LGTM.