Fix NPE in APMTracer through RestController (elastic#128314)

Our APMTracer doesn't like nulls - this is a sensible thing, as APM in general does not allow nulls (it only allows a precise set of types).
This PR changes the attribute to a sentinel "" in place of null values. It also makes a small change to APMTracer to give a better error message in case of null values in attributes.

Author: ldematte

docs/changelog/128314.yaml

@@ -0,0 +1,5 @@
+pr: 128314
+summary: Fix NPE in APMTracer through `RestController`
+area: Infra/REST API
+type: bug
+issues: []

modules/apm/src/main/java/org/elasticsearch/telemetry/apm/internal/tracing/APMTracer.java

@@ -304,6 +304,8 @@ private void setSpanAttributes(@Nullable Map<String, Object> spanAttributes, Spa
                     spanBuilder.setAttribute(key, (Double) value);
                 } else if (value instanceof Boolean) {
                     spanBuilder.setAttribute(key, (Boolean) value);
+                } else if (value == null) {
+                    throw new IllegalArgumentException("span attributes cannot have a null value");
                 } else {
                     throw new IllegalArgumentException(
                         "span attributes do not support value type of [" + value.getClass().getCanonicalName() + "]"

server/src/main/java/org/elasticsearch/rest/RestController.java

@@ -61,6 +61,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 import java.util.SortedMap;
 import java.util.TreeMap;
@@ -560,10 +561,10 @@ private void startTrace(ThreadContext threadContext, RestChannel channel, String
         final Map<String, Object> attributes = Maps.newMapWithExpectedSize(req.getHeaders().size() + 3);
         req.getHeaders().forEach((key, values) -> {
             final String lowerKey = key.toLowerCase(Locale.ROOT).replace('-', '_');
-            attributes.put("http.request.headers." + lowerKey, values.size() == 1 ? values.get(0) : String.join("; ", values));
+            attributes.put("http.request.headers." + lowerKey, values == null ? "" : String.join("; ", values));
         });
-        attributes.put("http.method", method);
-        attributes.put("http.url", req.uri());
+        attributes.put("http.method", Objects.requireNonNullElse(method, "<unknown>"));
+        attributes.put("http.url", Objects.requireNonNullElse(req.uri(), "<unknown>"));
         switch (req.getHttpRequest().protocolVersion()) {
             case HTTP_1_0 -> attributes.put("http.flavour", "1.0");
             case HTTP_1_1 -> attributes.put("http.flavour", "1.1");