feat: add OpenTelemetry tracing support behind otel feature flag

- Add tracing::instrument spans to executor (workflow.execute),
  host (workflow.start, event.publish, event.process)
- Add otel feature flag to wfe-core and wfe crates
- Add wfe/src/otel.rs helper for OTLP exporter initialization
- Dependencies: tracing-opentelemetry, opentelemetry, opentelemetry_sdk,
  opentelemetry-otlp (all optional behind otel feature)
- Step execution stays at info level, executor internals at debug

wfe/Cargo.toml

@@ -5,6 +5,17 @@ edition.workspace = true
 license.workspace = true
 description = "WFE workflow engine - umbrella crate"
 
+[features]
+default = []
+otel = [
+    "wfe-core/otel",
+    "opentelemetry",
+    "opentelemetry_sdk",
+    "opentelemetry-otlp",
+    "tracing-opentelemetry",
+    "tracing-subscriber/registry",
+]
+
 [dependencies]
 wfe-core = { workspace = true }
 tokio = { workspace = true }
@@ -19,6 +30,12 @@ tokio-util = "0.7"
 
 tracing-subscriber = { workspace = true }
 
+# OTel (optional, behind "otel" feature)
+tracing-opentelemetry = { workspace = true, optional = true }
+opentelemetry = { workspace = true, optional = true }
+opentelemetry_sdk = { workspace = true, optional = true }
+opentelemetry-otlp = { workspace = true, optional = true }
+
 [dev-dependencies]
 wfe-core = { workspace = true, features = ["test-support"] }
 wfe-sqlite = { workspace = true }

wfe/src/host.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use tokio::sync::RwLock;
 use tokio_util::sync::CancellationToken;
-use tracing::{debug, error, warn};
+use tracing::{debug, error, info, warn};
 
 use wfe_core::executor::{StepRegistry, WorkflowExecutor};
 use wfe_core::models::{
@@ -202,6 +202,15 @@ impl WorkflowHost {
     }
 
     /// Start a new workflow instance.
+    #[tracing::instrument(
+        name = "workflow.start",
+        skip(self, data),
+        fields(
+            workflow.definition_id = %definition_id,
+            workflow.version = version,
+            workflow.id,
+        )
+    )]
     pub async fn start_workflow(
         &self,
         definition_id: &str,
@@ -226,6 +235,9 @@ impl WorkflowHost {
         // Persist the instance.
         let id = self.persistence.create_new_workflow(&instance).await?;
         instance.id = id.clone();
+        tracing::Span::current().record("workflow.id", id.as_str());
+
+        info!(workflow_id = %id, "Workflow instance created");
 
         // Queue for execution.
         self.queue_provider
@@ -236,6 +248,14 @@ impl WorkflowHost {
     }
 
     /// Publish an event that may resume waiting workflows.
+    #[tracing::instrument(
+        name = "event.publish",
+        skip(self, data),
+        fields(
+            event.name = %event_name,
+            event.key = %event_key,
+        )
+    )]
     pub async fn publish_event(
         &self,
         event_name: &str,
@@ -312,6 +332,11 @@ impl WorkflowHost {
 }
 
 /// Process an event: find matching subscriptions, set event_data on pointers, re-queue workflows.
+#[tracing::instrument(
+    name = "event.process",
+    skip(persistence, lock_provider, queue),
+    fields(event.id = %event_id)
+)]
 async fn process_event(
     persistence: &Arc<dyn PersistenceProvider>,
     lock_provider: &Arc<dyn DistributedLockProvider>,