feat: add content-type checks for servlet instrumentations to ensure it's presence in span all times (#417)

* adding explicit checks for content-type

* nit fix

Author: thugrock7

instrumentation/servlet/servlet-3.0/src/main/java/io/opentelemetry/javaagent/instrumentation/hypertrace/servlet/v3_0/nowrapping/Servlet30AndFilterInstrumentation.java

@@ -186,6 +186,17 @@ public static void exit(
             if (throwable == null && !httpResponse.isCommitted()) {
               httpResponse.flushBuffer();
             }
+            if (httpResponse.getContentType() != null && !httpResponse.getContentType().isEmpty()) {
+              currentSpan.setAttribute(
+                  HypertraceSemanticAttributes.HTTP_RESPONSE_HEADER_CONTENT_TYPE,
+                  httpResponse.getContentType());
+            }
+            if (httpResponse.getCharacterEncoding() != null
+                && !httpResponse.getCharacterEncoding().isEmpty()) {
+              currentSpan.setAttribute(
+                  HypertraceSemanticAttributes.HTTP_RESPONSE_HEADER_CONTENT_ENCODING,
+                  httpResponse.getCharacterEncoding());
+            }
             for (String headerName : httpResponse.getHeaderNames()) {
               String headerValue = httpResponse.getHeader(headerName);
               currentSpan.setAttribute(

instrumentation/servlet/servlet-5.0/src/main/java/io/opentelemetry/javaagent/instrumentation/hypertrace/servlet/v5_0/Servlet50AndFilterInstrumentation.java

@@ -190,6 +190,17 @@ public static void exit(
             if (throwable == null && !httpResponse.isCommitted()) {
               httpResponse.flushBuffer();
             }
+            if (httpResponse.getContentType() != null && !httpResponse.getContentType().isEmpty()) {
+              currentSpan.setAttribute(
+                  HypertraceSemanticAttributes.HTTP_RESPONSE_HEADER_CONTENT_TYPE,
+                  httpResponse.getContentType());
+            }
+            if (httpResponse.getCharacterEncoding() != null
+                && !httpResponse.getCharacterEncoding().isEmpty()) {
+              currentSpan.setAttribute(
+                  HypertraceSemanticAttributes.HTTP_RESPONSE_HEADER_CONTENT_ENCODING,
+                  httpResponse.getCharacterEncoding());
+            }
             for (String headerName : httpResponse.getHeaderNames()) {
               String headerValue = httpResponse.getHeader(headerName);
               currentSpan.setAttribute(