histograms: configurable capacity

Log-linear histograms used in Envoy reserve 100 slots (each 10 bytes) for the sparse linked list by default. There's two of them per a thread-local histogram, meaning the space cost starts at 2kb per worker per histogram time series.

When the number of time series is very high (e.g. key-value tags), the samples are sparse, and 100 is likely over-capacity. It would make more sense to reduce the initial allocation to something lower.