test(pipeline): strengthen OTLP coverage and document header pairing

Address review-until-green feedback on the OTLP binding:
- Pin the default wire protocol: the endpoint-only OTLP test now asserts the
  request content-type is JSON (the http/json default) instead of accepting
  either json or protobuf.
- Cover multi-header export and the odd-length trailing-drop in one test (two
  header pairs plus a stray unpaired element; assert both pairs arrive).
- Document that setOtlpHeaders ignores a trailing unpaired element, replaces
  prior headers, and that setOtlpEndpoint takes precedence over setUseV05.

Author: bengl

crates/pipeline/src/lib.rs

@@ -302,6 +302,7 @@ impl WasmSpanState {
 
     /// Route trace export through libdatadog's OTLP HTTP exporter to `url`
     /// instead of the Datadog agent. Must be called before the first send.
+    /// Takes precedence over `setUseV05` (OTLP bypasses the agent entirely).
     #[wasm_bindgen(js_name = "setOtlpEndpoint")]
     pub fn set_otlp_endpoint(&self, url: String) {
         *self.otlp_endpoint.borrow_mut() = Some(url);
@@ -320,9 +321,13 @@ impl WasmSpanState {
     }
 
     /// Set extra HTTP headers for OTLP export as a flat `[key, value, ...]`
-    /// array. Only takes effect with an OTLP endpoint set, before the first send.
+    /// array (the host flattens its key/value map). Only takes effect with an
+    /// OTLP endpoint set, before the first send. A trailing unpaired element on
+    /// an odd-length array is ignored. Each call replaces any previously set headers.
     #[wasm_bindgen(js_name = "setOtlpHeaders")]
     pub fn set_otlp_headers(&self, kv: Vec<String>) {
+        // chunks_exact drops a trailing unpaired element; the host always passes
+        // complete [key, value] pairs.
         let headers = kv
             .chunks_exact(2)
             .map(|pair| (pair[0].clone(), pair[1].clone()))

test/pipeline.js

@@ -863,7 +863,8 @@ describe('pipeline', () => {
         const req = seen.find(r => r.method === 'POST')
         assert.ok(req, 'OTLP endpoint received a POST')
         assert.strictEqual(req.url, '/v1/traces')
-        assert.match(req.ct || '', /json|protobuf/)
+        // No setOtlpProtocol call — pins the default wire protocol (http/json).
+        assert.match(req.ct || '', /json/)
         assert.ok(req.len > 0, 'OTLP body is non-empty')
       } finally {
         server.closeAllConnections?.()
@@ -880,7 +881,8 @@ describe('pipeline', () => {
           if (req.method === 'POST') {
             captured = {
               ct: req.headers['content-type'],
-              auth: req.headers.authorization
+              auth: req.headers.authorization,
+              custom: req.headers['x-custom']
             }
           }
           res.writeHead(200, { 'content-type': 'application/json' })
@@ -892,7 +894,9 @@ describe('pipeline', () => {
       const ns = new NativeSpansInterface({ agentUrl: `http://127.0.0.1:${port}` })
       ns.state.setOtlpEndpoint(`http://127.0.0.1:${port}/v1/traces`)
       ns.state.setOtlpProtocol('http/protobuf')
-      ns.state.setOtlpHeaders(['authorization', 'Bearer test-token'])
+      // Two header pairs plus a trailing unpaired element (odd length): both
+      // pairs are applied and the stray 'ignored-no-pair' is dropped.
+      ns.state.setOtlpHeaders(['authorization', 'Bearer test-token', 'x-custom', 'cval', 'ignored-no-pair'])
       const span = ns.createSpan()
       span.name = 'otlp-span'
       span.service = 'test-service'
@@ -904,6 +908,7 @@ describe('pipeline', () => {
         assert.ok(captured, 'OTLP endpoint received a POST')
         assert.match(captured.ct || '', /protobuf/)
         assert.strictEqual(captured.auth, 'Bearer test-token')
+        assert.strictEqual(captured.custom, 'cval')
       } finally {
         server.closeAllConnections?.()
         server.close()