test: flaky TestOTLPGRPCLogsClientIP

From [CI logs](https://github.com/elastic/apm-server/actions/runs/27427590507/job/81069297292?pr=21157#step:6:253):
```
=== RUN TestOTLPGRPCLogsClientIP
otlp_test.go:508: []any{
map[string]any{
... // 2 identical entries
"agent.name.text": []any{string("android/java")},
"agent.version": []any{string("unknown")},
- "client.geo.city_name": []any{string("dynamic")},
- "client.geo.continent_name": []any{string("Europe")},
- "client.geo.country_iso_code": []any{string("SE")},
- "client.geo.country_name": []any{string("Sweden")},
- "client.geo.location": []any{string("dynamic")},
- "client.geo.region_iso_code": []any{string("dynamic")},
- "client.geo.region_name": []any{string("dynamic")},
"client.ip": []any{string("89.160.20.128")},
"data_stream.dataset": []any{string("apm.app.unknown")},
... // 19 identical entries
},
}

Test failed. Run `make check-approvals` to verify the diff.

server.go:144: log file: /home/runner/work/apm-server/apm-server/systemtest/logs/TestOTLPGRPCLogsClientIP/apm-server
```