perf(storage): memoize hashmap prom labels
perf(storage): memoize hashmap prom labels

Prior to this, each hashmap operation that was instrumented involved
initialising a map. Now these maps are pre-initialised.

```
⇒  benchstat old.txt new.txt
name                                                     old time/op    new time/op    delta
Index_CreateSeriesListIfNotExist/create_series-8            5.00s ± 3%     5.13s ± 2%   +2.50%  (p=0.033 n=10+7)
Index_CreateSeriesListIfNotExist/already_exist_series-8     557ms ± 3%     530ms ± 6%   -4.85%  (p=0.001 n=10+8)

name                                                     old alloc/op   new alloc/op   delta
Index_CreateSeriesListIfNotExist/create_series-8           2.57GB ± 0%    1.84GB ± 1%  -28.52%  (p=0.000 n=8+10)
Index_CreateSeriesListIfNotExist/already_exist_series-8     678MB ± 0%     308MB ± 0%  -54.55%  (p=0.000 n=10+8)

name                                                     old allocs/op  new allocs/op  delta
Index_CreateSeriesListIfNotExist/create_series-8            28.9M ± 0%     24.5M ± 0%  -15.22%  (p=0.000 n=9+10)
Index_CreateSeriesListIfNotExist/already_exist_series-8     2.23M ± 0%     0.03M ± 0%  -98.51%  (p=0.000 n=10+10)
```

influxdata/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,influxdata/influxdb,li-ang/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,nooproblem/influxdb,influxdb/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdb/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdb/influxdb