This commit adds a DynamicPrice member to the Services struct and uses
its values to determine if a GRPCPricer or DefaultPricer should be
initialised. The commit also updates the sample-conf.yaml file with the
new config options.
The service limiter holds all of the constraints that should be applied
to a given service at the base tier. These are currently static and are
parsed from the proxy's configuration file as a temporary work-around.
Eventually, we plan to integrate this with etcd as well in order to
achieve dynamic service discovery.
