WASM Support (#57)

## Motivation

I was attempting to use this library with a wasm target in a browser. If
you call `std::time::{Instant, SystemTime}::now` in a wasm target, it
will panic at runtime. This library makes use of both `::now()` calls,
which will result in a panic when attempting to record a trace.

## Solution

In the library that this library calls (`opentelemetry-api`), this is
resolved by replacing calls to `SystemTime::now()` with an
implementation that returns a `SystemTime` constructed from
`js_sys::Date`. To preserve the ability to pass `SystemTime`-typed args
to `opentelemetry_api` methods, I just added the same mechanism to this
library.

Because `Instant` is never used in `opentelemetry_api`, we can instead
replace the implementation entirely with the api-compatible "polyfill"
from `web_time`.

All of these changes are behind `cfg` and automatically enabled when
building for `wasm32` targets, so this will not change any
behavior/performance when built for other targets.

Cargo.toml

@@ -56,6 +56,10 @@ tracing-subscriber = { version = "0.3.0", default-features = false, features = [
 [target.'cfg(not(target_os = "windows"))'.dev-dependencies]
 pprof = { version = "0.11.1", features = ["flamegraph", "criterion"] }
 
+[target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dependencies]
+js-sys = "0.3.64"
+web-time = "0.2.0"
+
 [lib]
 bench = false
 

src/layer.rs

@@ -8,14 +8,17 @@ use std::any::TypeId;
 use std::fmt;
 use std::marker;
 use std::thread;
-use std::time::{Instant, SystemTime};
+#[cfg(not(target_arch = "wasm32"))]
+use std::time::Instant;
 use tracing_core::span::{self, Attributes, Id, Record};
 use tracing_core::{field, Event, Subscriber};
 #[cfg(feature = "tracing-log")]
 use tracing_log::NormalizeEvent;
 use tracing_subscriber::layer::Context;
 use tracing_subscriber::registry::LookupSpan;
 use tracing_subscriber::Layer;
+#[cfg(target_arch = "wasm32")]
+use web_time::Instant;
 
 const SPAN_NAME_FIELD: &str = "otel.name";
 const SPAN_KIND_FIELD: &str = "otel.kind";
@@ -680,7 +683,7 @@ where
         let mut builder = self
             .tracer
             .span_builder(attrs.metadata().name())
-            .with_start_time(SystemTime::now())
+            .with_start_time(crate::time::now())
             // Eagerly assign span id so children have stable parent id
             .with_span_id(self.tracer.new_span_id());
 
@@ -839,7 +842,7 @@ where
 
             let mut otel_event = otel::Event::new(
                 String::new(),
-                SystemTime::now(),
+                crate::time::now(),
                 vec![Key::new("level").string(meta.level().as_str()), target],
                 0,
             );
@@ -924,7 +927,7 @@ where
 
             // Assign end time, build and start span, drop span to export
             builder
-                .with_end_time(SystemTime::now())
+                .with_end_time(crate::time::now())
                 .start_with_context(&self.tracer, &parent_cx);
         }
     }

src/lib.rs

@@ -150,3 +150,17 @@ pub struct OtelData {
     /// The otel span data recorded during the current tracing span.
     pub builder: opentelemetry::trace::SpanBuilder,
 }
+
+pub(crate) mod time {
+    use std::time::SystemTime;
+
+    #[cfg(not(target_arch = "wasm32"))]
+    pub(crate) fn now() -> SystemTime {
+        SystemTime::now()
+    }
+
+    #[cfg(target_arch = "wasm32")]
+    pub(crate) fn now() -> SystemTime {
+        SystemTime::UNIX_EPOCH + std::time::Duration::from_millis(js_sys::Date::now() as u64)
+    }
+}