Allow disabling logging feature

[bobrik]

Logging macro if_log_enabled checks both log and always-log crate features to select which macro version to use. Unfortunately, log feature cannot be set by crate consumers in either existing form or if added to Cargo.toml, probably because of a name clash with log crate:

$ git diff Cargo.toml
diff --git a/tracing/Cargo.toml b/tracing/Cargo.toml
index 37bb61f..e030d27 100644
--- a/tracing/Cargo.toml
+++ b/tracing/Cargo.toml
@@ -63,7 +63,7 @@ release_max_level_trace = []
 async-await = []

 std = ["tracing-core/std"]
+log = ["log"]
 log-always = ["log"]
 attributes = ["tracing-attributes"]

Caused by:
  optional dependency `log` is not included in any feature
  Make sure that `dep:log` is included in one of features in the [features] table.

I'm renaming log feature to logging to work around that.

With the following code:

let now = std::time::Instant::now();

eprintln!("before span created us = {}", now.elapsed().as_micros());

let span = tracing::span!(tracing::Level::TRACE, "setup");

eprintln!("after span created us = {}", now.elapsed().as_micros());

let entered = span.enter();

eprintln!("after enter us = {}", now.elapsed().as_micros());

drop(entered);

eprintln!("after entered drop us = {}", now.elapsed().as_micros());

drop(span);

eprintln!("after span drop us = {}", now.elapsed().as_micros());

Typical timings with logging feature enabled (but not logging anything):

before span created us = 0
after span created us = 8
after enter us = 9
after entered drop us = 13
after span drop us = 14

Typical timings with logging disabled (this PR):

before span created us = 0
after span created us = 1
after enter us = 3
after entered drop us = 5
after span drop us = 6

[bobrik]

The timings are more different if I run the same traced code twice and measure more precisely, but it's still better.

fn trace_timings() {
    let now = std::time::Instant::now();

    let before_span_created = now.elapsed().as_nanos();

    let span = tracing::span!(tracing::Level::TRACE, "setup");

    let after_span_created = now.elapsed().as_nanos();

    let entered = span.enter();

    let after_span_entered = now.elapsed().as_nanos();

    drop(entered);

    let after_span_entered_dropped = now.elapsed().as_nanos();

    drop(span);

    let after_span_dropped = now.elapsed().as_nanos();

    eprintln!("before span created ns = {:6}", before_span_created);
    eprintln!("after span created ns  = {:6}", after_span_created);
    eprintln!("after enter ns         = {:6}", after_span_entered);
    eprintln!("after entered drop ns  = {:6}", after_span_entered_dropped);
    eprintln!("after span drop ns     = {:6}", after_span_dropped);
}

• Before (logging feature on, no logs produced):

before span created ns =      0
after span created ns  =   5833
after enter ns         =   6041
after entered drop ns  =  10375
after span drop ns     =  10583
---
before span created ns =      0
after span created ns  =    125
after enter ns         =    166
after entered drop ns  =    250
after span drop ns     =    291

• After (logging feature off):

before span created ns =      0
after span created ns  =    333
after enter ns         =   1333
after entered drop ns  =   2875
after span drop ns     =   3125
---
before span created ns =      0
after span created ns  =      0
after enter ns         =     41
after entered drop ns  =     83
after span drop ns     =    125

[bobrik]

Please disregard my ramblings about the timings, these are from a non-release build.

Actual difference from the benchmark:

global/collector/span_no_fields
                        time:   [7.9682 ns 7.9704 ns 7.9729 ns]
                        change: [-13.902% -13.812% -13.720%] (p = 0.00 < 0.05)
                        Performance has improved.

I amended the commit to reflect that.

#1191 should probably be merged before this.

[hawkw]

Unfortunately, log feature cannot be set by crate consumers in either existing form or if added to Cargo.toml, probably because of a name clash with log crate:

I'm not sure if this is the case. It should be possible to enable or disable the optional dependency as though it were a crate feature, without adding it to the tracing Cargo.toml as a feature flag.

Something like this should work, and I'm pretty sure it's been used successfully in the past.

[dependencies.tracing]
version = "0.1"
features = ["log"]

Does this no longer work?

[bobrik]

Not sure how I messed up my local setup, but it does indeed work as intended.