Add size metric for head types

[simonswine]

Keeps head metrics per type

Example:

# HELP fire_head_profiles_created_total Total number of profiles created in the head
# TYPE fire_head_profiles_created_total counter
fire_head_profiles_created_total 83
# HELP fire_head_size_bytes Size of a particular in memory store within the head firedb block.
# TYPE fire_head_size_bytes gauge
fire_head_size_bytes{type="functions"} 45200
fire_head_size_bytes{type="mappings"} 672
fire_head_size_bytes{type="pprof-labels"} 4176
fire_head_size_bytes{type="profiles"} 236856
fire_head_size_bytes{type="stacktraces"} 41544
fire_head_size_bytes{type="strings"} 42688

Fixes #71

[cyriltovena]

LGTM