TestDefaultMonitoringMapForRegistry is flaky on rerun

[kruskall]

APM Server version (apm-server version): 8.10

Description of the problem including expected versus actual behavior:

TestDefaultMonitoringMapForRegistry fails when running tests multiple times

Steps to reproduce:

Please include a minimal but complete recreation of the problem,
including server configuration, agent(s) used, etc. The easier you make it
for us to reproduce it, the more likely that somebody will take the time to
look at it.

1. go test -run=TestDefaultMonitoringMapForRegistry -race -count=100 -failfast ./...

Provide logs (if relevant):

--- FAIL: TestDefaultMonitoringMapForRegistry (0.00s)
panic: name mock-default already used [recovered]
        panic: name mock-default already used

goroutine 20 [running]:
testing.tRunner.func1.2({0x8cb840, 0xc0002bd260})
        /usr/lib/go/src/testing/testing.go:1526 +0x372
testing.tRunner.func1()
        /usr/lib/go/src/testing/testing.go:1529 +0x650
panic({0x8cb840, 0xc0002bd260})
        /usr/lib/go/src/runtime/panic.go:890 +0x263
github.com/elastic/elastic-agent-libs/monitoring.panicErr(...)
        /go/pkg/mod/github.com/elastic/elastic-agent-libs@v0.3.9/monitoring/registry.go:257
github.com/elastic/elastic-agent-libs/monitoring.(*Registry).Add(0xc00023c780, {0x93b176, 0xc}, {0x9dcc00, 0xc00023d2c0}, 0x1)
        /go/pkg/mod/github.com/elastic/elastic-agent-libs@v0.3.9/monitoring/registry.go:155 +0x19b
github.com/elastic/elastic-agent-libs/monitoring.(*Registry).NewRegistry(0xc00023c780, {0x93b176, 0xc}, {0x0, 0x0, 0x0})
        /go/pkg/mod/github.com/elastic/elastic-agent-libs@v0.3.9/monitoring/registry.go:94 +0x298
github.com/elastic/apm-server/internal/beater/request.TestDefaultMonitoringMapForRegistry(0x0?)
        /apm-server/internal/beater/request/result_test.go:193 +0x69
testing.tRunner(0xc0002ded00, 0x96c858)
        /usr/lib/go/src/testing/testing.go:1576 +0x217
created by testing.(*T).Run
        /usr/lib/go/src/testing/testing.go:1629 +0x806