Add supportability metric for agent version

[ohinton]

Is your feature request related to a problem? Please describe.

When a customer reports an issue in a support case, a troubleshooting step I take when supporting the .NET agent is to go the application's Metric Explorer view in order to check if the customer's issue might be related to an agent version change in the past.

For example, the .NET agent has a supportability metric that looks like this:
Supportability/AgentVersion/[AGENT VERSION]

Currently an application's environment page only displays the current agent version being reported, and there is not an easy way to see past agent versions and when an agent version was changed.

It is helpful to have evidence of an agent version change around the time an issue is described as happening, because sometimes customers are not aware that an agent change happened.

Feature Description

Add a supportability metric for the agent version, which can be viewed in the Metric Explorer and used for as a resource when troubleshooting customer issues.

Priority

Nice to Have

[workato-integration]

https://new-relic.atlassian.net/browse/NR-266691

[kaylareopelle]

Thank you, @ohinton!

Looking at Angler, Java has a similar metric: Supportability/Java/AgentVersion/<version>

Mobile has similar metrics for Android and IOS:
Supportability/Mobile/AgentVersion/AndroidAgent/<version>
Supportability/Mobile/AgentVersion/iOSAgent/<version>

I don't see similar metrics for Python, Node, Go, or PHP in Angler.

We may want to consider an agent spec update as part of this change to provide the metric consistently across all language agents.