Add a counted procedural macro to simplify handling count metrics

Author: manuelgdlvh

Cargo.toml

@@ -48,3 +48,5 @@ tracing = { version = "0.1", default-features = false }
 tracing-core = { version = "0.1", default-features = false }
 tracing-subscriber = { version = "0.3", default-features = false }
 url = { version = "2.5", default-features = false }
+syn = { version = "2.0.72", fedefault-features = false, features = ["full"] }
+

opentelemetry/Cargo.toml

@@ -26,6 +26,8 @@ futures-sink = "0.3"
 once_cell = { workspace = true }
 pin-project-lite = { workspace = true, optional = true }
 thiserror = { workspace = true }
+syn = { workspace = true }
+
 
 [target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dependencies]
 js-sys = "0.3.63"
@@ -40,10 +42,15 @@ logs_level_enabled = ["logs"]
 otel_unstable = []
 
 [dev-dependencies]
-opentelemetry_sdk = { path = "../opentelemetry-sdk", features = ["logs_level_enabled"]} # for documentation tests
+opentelemetry_sdk = { path = "../opentelemetry-sdk", features = ["logs_level_enabled"] } # for documentation tests
 criterion = { workspace = true }
 rand = { workspace = true }
 
+[lib]
+name = "macros"
+proc-macro = true
+path = "src/macros.rs"
+
 [[bench]]
 name = "metrics"
 harness = false

opentelemetry/src/macros.rs

@@ -0,0 +1,58 @@
+use proc_macro::TokenStream;
+use syn::{Block, ItemFn, LitStr, parse_macro_input};
+use syn::__private::ToTokens;
+
+// Default Values
+const DEFAULT_METER_PROVIDER_NAME: &'static str = "default_meter_provider";
+const DEFAULT_DESCRIPTION: &'static str = "Empty description!";
+
+// Attributes
+const METER_PROVIDER_NAME_ATTR_NAME: &'static str = "meter_provider";
+const NAME_ATTR_NAME: &'static str = "name";
+const DESCRIPTION_ATTR_NAME: &'static str = "description";
+
+// Messages
+const ATTR_ERROR_MESSAGE: &'static str = "unsupported attribute for counted macro!";
+
+
+#[proc_macro_attribute]
+pub fn counted(attr: TokenStream, item: TokenStream) -> TokenStream {
+    let mut input = parse_macro_input!(item as ItemFn);
+
+    let mut name = format!("fn_{}_count", input.sig.ident.to_string());
+    let mut meter_provider = DEFAULT_METER_PROVIDER_NAME.to_string();
+    let mut description = DEFAULT_DESCRIPTION.to_string();
+    let parser = syn::meta::parser(|meta| {
+        if meta.path.is_ident(NAME_ATTR_NAME) {
+            let value = meta.value()?.parse::<LitStr>()?;
+            name = value.value();
+            return Ok(());
+        } else if meta.path.is_ident(METER_PROVIDER_NAME_ATTR_NAME) {
+            let value = meta.value()?.parse::<LitStr>()?;
+            meter_provider = value.value();
+            return Ok(());
+        } else if meta.path.is_ident(DESCRIPTION_ATTR_NAME) {
+            let value = meta.value()?.parse::<LitStr>()?;
+            description = value.value();
+            return Ok(());
+        }
+
+        Err(meta.error(ATTR_ERROR_MESSAGE))
+    });
+
+    parse_macro_input!(attr with parser);
+
+    let mut counted_code_block = syn::parse_str::<Block>(&format!(r#"
+            {{
+                static COUNTER: std::sync::LazyLock<opentelemetry::metrics::Counter<u64>> = std::sync::LazyLock::new(|| {{
+                let meter = opentelemetry::global::meter("{}");
+                meter.u64_counter("{}").with_description("{}").init()
+            }});
+                COUNTER.add(1, &[]);
+            }}
+    "#, meter_provider, name, description)).unwrap();
+
+    counted_code_block.stmts.extend_from_slice(&*input.block.stmts);
+    input.block.stmts = counted_code_block.stmts;
+    TokenStream::from(input.into_token_stream())
+}