call AgentHealthServiceByName get error

```
status, services, err := client.Agent().AgentHealthServiceByName(serviceName)
if err != nil {
log.Errorf("failed to get service %s: %v", serviceName, err)
return
}
```

after parallel call 1000 times in a second between service a and b, will got following err:
```
ERROR: failed to get service a: invalid character 'Y' looking for beginning of value
ERROR: failed to get service b: invalid character 'Y' looking for beginning of value
```

service will resume after a while.

Start at the AgentHealthServiceByName entry point and reproduce the reported parallel calls between services a and b. Inspect how the client handles responses under concurrency; done means the calls no longer produce invalid JSON errors during the reported load.

Written by the indexing model from the issue text.