storage_proxy: measure latency for failed writes too
storage_proxy: measure latency for failed writes too

This was the case before 0149a22f693aab23ee45108af4340e33650c6e2f

shaunstanislaus/scylla,dwdm/scylla,gwicke/scylla,capturePointer/scylla,justintung/scylla,stamhe/scylla,kjniemi/scylla,bowlofstew/scylla,tempbottle/scylla,capturePointer/scylla,acbellini/scylla,senseb/scylla,linearregression/scylla,wildinto/scylla,rentongzhang/scylla,victorbriz/scylla,capturePointer/scylla,scylladb/scylla,phonkee/scylla,duarten/scylla,shaunstanislaus/scylla,tempbottle/scylla,linearregression/scylla,dwdm/scylla,kjniemi/scylla,rentongzhang/scylla,aruanruan/scylla,eklitzke/scylla,glommer/scylla,senseb/scylla,phonkee/scylla,avikivity/scylla,scylladb/scylla,glommer/scylla,stamhe/scylla,respu/scylla,raphaelsc/scylla,phonkee/scylla,eklitzke/scylla,raphaelsc/scylla,scylladb/scylla,respu/scylla,aruanruan/scylla,respu/scylla,asias/scylla,kangkot/scylla,victorbriz/scylla,justintung/scylla,rluta/scylla,rluta/scylla,wildinto/scylla,guiquanz/scylla,avikivity/scylla,glommer/scylla,rentongzhang/scylla,asias/scylla,dwdm/scylla,asias/scylla,linearregression/scylla,bowlofstew/scylla,kangkot/scylla,senseb/scylla,kangkot/scylla,shaunstanislaus/scylla,kjniemi/scylla,scylladb/scylla,wildinto/scylla,rluta/scylla,duarten/scylla,duarten/scylla,bowlofstew/scylla,guiquanz/scylla,acbellini/scylla,acbellini/scylla,raphaelsc/scylla,guiquanz/scylla,avikivity/scylla,gwicke/scylla,aruanruan/scylla,stamhe/scylla,tempbottle/scylla,gwicke/scylla,justintung/scylla,victorbriz/scylla,eklitzke/scylla