Make metrics disablable.

Beyond uses sigar for metrics, but there is a problem to use sigar
library in production mode. So disable it until fix the problem.

Enabling metrics are determined by `enable-metrics` option.

And this patch disables metrics only for production mode.
Handle ConsString for JSON body in ScriptableHttpClient.

Currently a convert function in ScriptableHttpClient doesn't handle the
ConsString object. This patch fixes the module to handle it.