mozzle: Emit both client and server HTTP metrics

The firehose provides metrics for communication between the CF
router and the application instances, and the end user and CF
router. Now both metrics are propagated via mozzle.

The way to distinguish them is via the 'peer' metric attribute,
which has value of either 'server' or 'client'. The attribute
describes where the metrics are actually measured:

      HTTP          HTTP
user <----> router <----> application instance
     server        client

  * server - metrics are measured as the router is writing the
    response back to the client.
  * client - metrics are measured as the router is requesting an
    application instance.

Fixes #7.