test: flaky TestAddExpvarMetrics

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
```

Start with systemtest/benchtest/main_test.go:220 and the TestAddExpvarMetrics subtest shown in the CI failure. Run the test repeatedly or inspect its setup to determine why events/sec is sometimes 0 instead of 10; done means the test reliably passes for all listed flag and response cases.

Written by the indexing model from the issue text.