log internal metrics to a drain, if configured
add branch and az to internal metric logs
add internal_drain_token to env list
properly assemble messages for internal metrics

construct messages using logplex_syslog_utils:to_msg()
handle drain lookup as list or binary
cleanup list/binary -> integer conversion (thanks @tsloughter)
internal_drain_token -> internal_metrics_channel_token
this should make INTERNAL_METRICS_CHANNEL_TOKEN optional
separate function for publishing to metrics channel
no need to match channel token format here
correct config name in readme
Clean up metric msg formatting, skip tails + redis

Rather than sending these logs through the entire message pipeline, skip
to injecting them directly in the drain stream. Avoid putting them into
Redis or sending them to tails.
use channel ID instead of token for metrics

For internal metrics, avoid having to lookup the channel ID from the
token by just starting with the token.
Add a way to specify that a config is an integer

Use that convention for internal_metrics_channel_id
minor style changes
Merge pull request #31 from heroku/l2met-visibility

Add L2met style metrics channel/drain to Logplex.