Always allow sample size for stacktraces (elastic#103101)

The profiling get stacktraces API can be used in two different places:

1. In the native profiling UI
2. From within an APM context

In the latter case we disallowed the `sample_size` request body
parameter because the implementation always considers all samples.
However, as this will change in the future and we need to accept the
`sample_size` parameter this will lead to rejected requests during
upgrades. With this commit we always accept the `sample_size` request
body parameter to avoid such cases.

x-pack/plugin/profiling/src/main/java/org/elasticsearch/xpack/profiling/GetStackTracesRequest.java

@@ -247,22 +247,15 @@ public ActionRequestValidationException validate() {
                     validationException
                 );
             }
-            // we don't do downsampling when a custom index is provided
-            if (sampleSize != null) {
-                validationException = addValidationError(
-                    "[" + SAMPLE_SIZE_FIELD.getPreferredName() + "] must not be set",
-                    validationException
-                );
-            }
         } else {
             if (stackTraceIds != null) {
                 validationException = addValidationError(
                     "[" + STACKTRACE_IDS_FIELD.getPreferredName() + "] must not be set",
                     validationException
                 );
             }
-            validationException = requirePositive(SAMPLE_SIZE_FIELD, sampleSize, validationException);
         }
+        validationException = requirePositive(SAMPLE_SIZE_FIELD, sampleSize, validationException);
         validationException = requirePositive(REQUESTED_DURATION_FIELD, requestedDuration, validationException);
         validationException = requirePositive(AWS_COST_FACTOR_FIELD, awsCostFactor, validationException);
         validationException = requirePositive(CUSTOM_CO2_PER_KWH, customCO2PerKWH, validationException);

x-pack/plugin/profiling/src/test/java/org/elasticsearch/xpack/profiling/GetStackTracesRequestTests.java

@@ -170,6 +170,23 @@ public void testValidateWrongSampleSize() {
         assertTrue(validationErrors.get(0).contains("[sample_size] must be greater than 0,"));
     }
 
+    public void testValidateSampleSizeIsValidWithCustomIndices() {
+        GetStackTracesRequest request = new GetStackTracesRequest(
+            10,
+            1.0d,
+            1.0d,
+            null,
+            randomAlphaOfLength(7),
+            randomAlphaOfLength(3),
+            null,
+            null,
+            null,
+            null,
+            null
+        );
+        assertNull("Expecting no validation errors", request.validate());
+    }
+
     public void testValidateStacktraceWithoutIndices() {
         GetStackTracesRequest request = new GetStackTracesRequest(
             1,