Fix vault field name (elastic#129184)

Author: idegtiarenko

.buildkite/hooks/pre-command

@@ -96,7 +96,7 @@ fi
 
 if [[ "${USE_PERF_CREDENTIALS:-}" == "true" ]]; then
   PERF_METRICS_HOST=$(vault read -field=es_host /secret/ci/elastic-elasticsearch/microbenchmarks-metrics)
-  PERF_METRICS_USERNAME=$(vault read -field=es_username /secret/ci/elastic-elasticsearch/microbenchmarks-metrics)
+  PERF_METRICS_USERNAME=$(vault read -field=es_user /secret/ci/elastic-elasticsearch/microbenchmarks-metrics)
   PERF_METRICS_PASSWORD=$(vault read -field=es_password /secret/ci/elastic-elasticsearch/microbenchmarks-metrics)
 
   export PERF_METRICS_HOST