feat: produce measurement with attributes (tokio-rs#43)

## Motivation

Support tokio-rs#32
This change will allow user to associate Attributes with metrics from
tracing.

## Solution

1. MetricsLayer is only interested in events that contain fields with
metrics prefixes such as `counter`, so I have utilized the
[tracing_subscriber Filter
trait](https://docs.rs/tracing-subscriber/latest/tracing_subscriber/layer/#per-layer-filtering).
2. Since it's necessary to pass Attributes when outputting Measurements,
it's required to complete the visit for all fields first before invoking
the Metrics API(`Counter:add()`).
`SmallVec` is used to hold Attribute KeyValues, Metrics names, and
Instruments for invocation, aiming to avoid allocations in typical use
cases.

Cargo.toml

@@ -22,7 +22,7 @@ rust-version = "1.60.0"
 [features]
 default = ["tracing-log", "metrics"]
 # Enables support for exporting OpenTelemetry metrics
-metrics = ["opentelemetry/metrics"]
+metrics = ["opentelemetry/metrics", "smallvec"]
 
 [dependencies]
 opentelemetry = { version = "0.20.0", default-features = false, features = ["trace"] }
@@ -35,6 +35,7 @@ once_cell = "1.13.0"
 # Fix minimal-versions
 async-trait = { version = "0.1.56", optional = true }
 thiserror = { version = "1.0.31", optional = true }
+smallvec = { version = "1.0", optional = true }
 
 [dev-dependencies]
 async-trait = "0.1.56"
@@ -56,6 +57,10 @@ bench = false
 name = "trace"
 harness = false
 
+[[bench]]
+name = "metrics"
+harness = false
+
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]

benches/metrics.rs

@@ -0,0 +1,162 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use opentelemetry::metrics::noop::NoopMeterProvider;
+use tracing_opentelemetry::MetricsLayer;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
+
+fn metrics_events(c: &mut Criterion) {
+    let mut group = c.benchmark_group("otel_metrics_events");
+    {
+        let _subscriber = tracing_subscriber::registry().set_default();
+        group.bench_function("no_metrics_layer", |b| {
+            b.iter(|| {
+                tracing::info!(key_1 = "va", "msg");
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_0_attr_0", |b| {
+            b.iter(|| {
+                tracing::info!(key_1 = "va", "msg");
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_1_attr_0", |b| {
+            b.iter(|| {
+                tracing::info!(monotonic_counter.c1 = 1, "msg");
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_2_attr_0", |b| {
+            b.iter(|| {
+                tracing::info!(monotonic_counter.c1 = 1, monotonic_counter.c2 = 1, "msg");
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_4_attr_0", |b| {
+            b.iter(|| {
+                tracing::info!(
+                    monotonic_counter.c1 = 1,
+                    monotonic_counter.c2 = 1,
+                    monotonic_counter.c3 = 1,
+                    monotonic_counter.c4 = 1,
+                    "msg"
+                );
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_8_attr_0", |b| {
+            b.iter(|| {
+                tracing::info!(
+                    monotonic_counter.c1 = 1,
+                    monotonic_counter.c2 = 1,
+                    monotonic_counter.c3 = 1,
+                    monotonic_counter.c4 = 1,
+                    monotonic_counter.c5 = 1,
+                    monotonic_counter.c6 = 1,
+                    monotonic_counter.c7 = 1,
+                    monotonic_counter.c8 = 1,
+                    "msg"
+                );
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_1_attr_1", |b| {
+            b.iter(|| {
+                tracing::info!(monotonic_counter.c1 = 1, key_1 = 1_i64, "msg");
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_1_attr_2", |b| {
+            b.iter(|| {
+                tracing::info!(
+                    monotonic_counter.c1 = 1,
+                    key_1 = 1_i64,
+                    key_2 = 1_i64,
+                    "msg"
+                );
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_1_attr_4", |b| {
+            b.iter(|| {
+                tracing::info!(
+                    monotonic_counter.c1 = 1,
+                    key_1 = 1_i64,
+                    key_2 = 1_i64,
+                    key_3 = 1_i64,
+                    key_4 = 1_i64,
+                    "msg"
+                );
+            })
+        });
+    }
+
+    {
+        let _subscriber = tracing_subscriber::registry()
+            .with(MetricsLayer::new(NoopMeterProvider::new()))
+            .set_default();
+        group.bench_function("metrics_events_1_attr_8", |b| {
+            b.iter(|| {
+                tracing::info!(
+                    monotonic_counter.c1 = 1,
+                    key_1 = 1_i64,
+                    key_2 = 1_i64,
+                    key_3 = 1_i64,
+                    key_4 = 1_i64,
+                    key_5 = 1_i64,
+                    key_6 = 1_i64,
+                    key_7 = 1_i64,
+                    key_8 = 1_i64,
+                    "msg"
+                );
+            })
+        });
+    }
+    group.finish();
+}
+
+criterion_group! {
+    name = benches;
+    config = Criterion::default();
+    targets = metrics_events
+}
+criterion_main!(benches);