Add metrics on the agent that can show connections status between agent with collection

[WalkerWang731]

Requirement - what kind of business use case are you trying to solve?

Add metrics on the agent that can show connections status between the agent with collections.
We need monitoring of agent connection status in the production environment to ensure the agent connected.
But, so far, we can't get these metrics via http://jaeger-agent:14271/metrics, especially when the agent disconnected collections, current metrics not provide clear metrics to discover.
So I thought to provide this metric.

Problem - what in Jaeger blocks you from solving the requirement?

Not metrics to discover this case

When the agent disconnected, just only output the error log and can't provide relevant metrics. Make engineers in an unstable environment.

Proposal - what do you suggest to solve the problem or improve the existing situation?

As gRPC connects or changes metrics also change

Like below:

# HELP jaeger_agent_connection_status_connected_collector_status Connection status that jaeger-agent to jaeger-collector, 1 is connected, 0 is disconnected
# TYPE jaeger_agent_connection_status_connected_collector_status gauge
jaeger_agent_connection_status_connected_collector_status{protocol="grpc",target="127.0.0.1:14250"} 1

Any open questions to address

I had solved the issue, let me create a pull request