Add metrics `LoggingBackend`.
Add metrics `LoggingBackend`.

This make it easier to understand during development what metrics are
defined.

This can be enabled in the Sentry configuration by updating the
`SENTRY_METRICS_BACKEND` and updating the `LOGGING` configuration:

    SENTRY_METRICS_BACKEND = 'sentry.metrics.logging.LoggingBackend'

    LOGGING['handlers']['console:metrics'] = {
        'level': 'DEBUG',
        'class': 'logging.StreamHandler',
        'formatter': 'simple',
    }

    LOGGING['loggers']['sentry.metrics'] = {
        'level': 'DEBUG',
        'handlers': ['console:metrics'],
        'propagate': False,
    }

gencer/sentry,daevaorn/sentry,zenefits/sentry,fotinakis/sentry,looker/sentry,ifduyue/sentry,zenefits/sentry,mvaled/sentry,JamesMura/sentry,beeftornado/sentry,jean/sentry,looker/sentry,mvaled/sentry,mvaled/sentry,zenefits/sentry,alexm92/sentry,daevaorn/sentry,fotinakis/sentry,alexm92/sentry,JamesMura/sentry,gencer/sentry,zenefits/sentry,jean/sentry,imankulov/sentry,jean/sentry,mvaled/sentry,imankulov/sentry,nicholasserra/sentry,jean/sentry,zenefits/sentry,mvaled/sentry,nicholasserra/sentry,fotinakis/sentry,fotinakis/sentry,mvaled/sentry,ifduyue/sentry,BuildingLink/sentry,gencer/sentry,mitsuhiko/sentry,BuildingLink/sentry,beeftornado/sentry,BuildingLink/sentry,BayanGroup/sentry,nicholasserra/sentry,BuildingLink/sentry,looker/sentry,daevaorn/sentry,gencer/sentry,daevaorn/sentry,ifduyue/sentry,ifduyue/sentry,JackDanger/sentry,mitsuhiko/sentry,jean/sentry,ifduyue/sentry,BayanGroup/sentry,JamesMura/sentry,alexm92/sentry,gencer/sentry,BuildingLink/sentry,looker/sentry,JackDanger/sentry,JamesMura/sentry,beeftornado/sentry,JamesMura/sentry,BayanGroup/sentry,imankulov/sentry,looker/sentry,JackDanger/sentry