fix(http-client): use lowercase header key for Content-Length lookup

Symfony's getHeaders() normalizes header names to lowercase, so the
Content-Length check was unreachable. Access the first array element
since headers are returned as arrays.

Author: gaelreyrol

src/Instrumentation/Symfony/HttpClient/TraceableResponse.php

@@ -138,8 +138,8 @@ private function endSpan(): void
         $statusCode = $this->response->getStatusCode();
         if (0 !== $statusCode && $this->span->isRecording()) {
             $headers = $this->response->getHeaders(false);
-            if (isset($headers['Content-Length'])) {
-                $this->span->setAttribute(HttpIncubatingAttributes::HTTP_RESPONSE_BODY_SIZE, $headers['Content-Length']);
+            if (isset($headers['content-length'][0])) {
+                $this->span->setAttribute(HttpIncubatingAttributes::HTTP_RESPONSE_BODY_SIZE, $headers['content-length'][0]);
             }
 
             $this->span->setAttribute(HttpAttributes::HTTP_RESPONSE_STATUS_CODE, $statusCode);

tests/Functional/Instrumentation/HttpClientTracingTest.php

@@ -29,7 +29,7 @@ public function testOk(): void
                 'url' => 'http://localhost/ok',
                 'http_code' => 200,
                 'http_method' => 'GET',
-                'response_headers' => ['Content-Type' => 'application/json'],
+                'response_headers' => ['Content-Type' => 'application/json', 'Content-Length' => '16'],
             ])),
             self::getContainer()->get('open_telemetry.traces.tracers.main'),
             new Psr17Factory(),
@@ -39,7 +39,7 @@ public function testOk(): void
 
         self::assertSame(Response::HTTP_OK, $response->getStatusCode());
         self::assertSame('{"status": "ok"}', $response->getContent());
-        self::assertSame(['content-type' => ['application/json']], $response->getHeaders());
+        self::assertSame(['content-type' => ['application/json'], 'content-length' => ['16']], $response->getHeaders());
 
         self::assertSpansCount(1);
 
@@ -53,6 +53,7 @@ public function testOk(): void
             'url.query' => '',
             'url.fragment' => '',
             'http.request.method' => 'GET',
+            'http.response.body.size' => '16',
             'http.response.status_code' => 200,
         ]);
         self::assertSpanEventsCount($mainSpan, 0);