Add stats_hero_label/1 and stats_hero_upstreams/0 to chef_wb_base

The stats_hero_label function will be used for stats_hero
instrumentation to generate labels with appropriate prefix for
aggregation.

The behavior is slightly changed from the previous implementation that
lived in the stats_hero module. There, we transformed {Mod, Fun} to
Prefix.Fun. That lost the module information for no benefit. This
function transforms to Prefix.Mod.Fun which should behave the same way
in terms of aggregation, but allows finer grained data to be accessed
if we so choose.

The stats_hero_upstreams function is used for configuring stats_hero
with the list of upstream prefixes to use for metrics aggregation.
Log inability to start stats_hero worker, but continue
Add metrics_config field to base_state record

Configuration for the top-level application name for metrics is now
retrieved from chef_wm app config. The stats_hero label function and
list of upstreams are obtained via ?BASE_RESOURCE. This isn't fully
flexibly, but it isn't clear if there would be value in exposing this
config all the way into app config. All three values are put into a
proplist and stored in the base_state record in the metrics_config
field.
Merge branch 'sf/new_stats_hero'