Provide a way to log metrics via context.log

[jeffhollan]

In C# you can use the logger to create custom metrics in App Insights. Would be nice to expose this as well - potentially with something like context.log.metric('Duration', 1234)

[mhoeger]

(Note: this feature is available in V1 but not V2)

[ejizba]

The link to the docs in the original issue is out of date, and I think by now all languages (except maybe C#Script?) are leveraging an app insights sdk as opposed to something on the logger. We have several improvements around app insights already on the docket, namely #522 and #523. Closing in favor of those and due to lack of activity on this issue