only create qps client when live metrics is enabled

[markwolff]

fixes #612

[leidegre]

I would like to add that I think it it important that you include that this change actually fixes an actual memory leak.

This leak took down our production environment and caused an outage for about 2 hours. I spent the last week trying to root cause the issue and all that I did to deserve this was that I used the default config as per the examples given for the package.

Now that it has been fixed, it is less likely that newcomers will run into this problem but every system out there, that is running the older code, and using the example as a starting point is leaking memory in such a way that the system will degrade to a point where it will run out of memory completely and fail. I think it deserves some kind of special mention.