test: flaky TestAddExpvarMetrics

[carsonip]

https://github.com/elastic/apm-server/actions/runs/27567049707/job/81493620403?pr=21208

[37](https://github.com/elastic/apm-server/actions/runs/27567049707/job/81493620403?pr=21208#step:6:338)
=== RUN   TestAddExpvarMetrics
=== RUN   TestAddExpvarMetrics/with_false_detailed_flag_and_no_error_resp
    main_test.go:220: 
        	Error Trace:	/home/runner/work/apm-server/apm-server/systemtest/benchtest/main_test.go:220
        	Error:      	Not equal: 
        	            	expected: map[string]float64{"events/sec":10}
        	            	actual  : map[string]float64{"events/sec":0}
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1,3 +1,3 @@
        	            	 (map[string]float64) (len=1) {
        	            	- (string) (len=10) "events/sec": (float64) 10
        	            	+ (string) (len=10) "events/sec": (float64) 0
        	            	 }
        	Test:       	TestAddExpvarMetrics/with_false_detailed_flag_and_no_error_resp
=== RUN   TestAddExpvarMetrics/with_false_detailed_flag_and_error_resp
=== RUN   TestAddExpvarMetrics/with_true_detailed_flag_and_error_resp
--- FAIL: TestAddExpvarMetrics (0.40s)
    --- FAIL: TestAddExpvarMetrics/with_false_detailed_flag_and_no_error_resp (0.14s)
    --- PASS: TestAddExpvarMetrics/with_false_detailed_flag_and_error_resp (0.14s)
    --- PASS: TestAddExpvarMetrics/with_true_detailed_flag_and_error_resp (0.12s)
FAIL
FAIL	github.com/elastic/apm-server/systemtest/benchtest	12.688s