Add metrics counters

[icebob]

Add following metrics:

• [x] moleculer.channels.messages.sent with labels: channel _sent messages via sendToChannel
• [x] moleculer.channels.messages.total with labels: channel, group _total processed messages in channel handlers`
• [x] moleculer.channels.messages.active with labels: channel, group _total active processing messages in channel handlers (gauge)`
• [x] moleculer.channels.messages.time with labels: channel, group _histogram for processing time of channel handlers`
• [ ] moleculer.channels.messages.errors.total with labels: channel, group _total errored messages in channel handlers`
• [ ] moleculer.channels.messages.retries.total with labels: channel, group _total retried messages in channel handlers`
• [ ] moleculer.channels.messages.deadLettering.total with labels: channel, group _total dead-lettered messages in channel handlers`