Add user-friendly macros to support optional properties, tags

[ranweiler]

As a user, I'd love a log/tracing-style mechanism where I can opt into initializing and registering a global client, and then using macros to simulate variadic calls that let me insert custom tags and properties.

In practice, this could look like:

appinsights::track_event!("my custom event"; k1 = "v1", k2 = ?v2, k3 = %v3);

In this example, the event name would be custom event, k1 would map to the string v1, k2 would map to the Debug impl for the value of v2, and k3 would map to the Display impl for the value of v3. This could (ideally) be supported in all event types.

[dmolokanov]

Totally agree. Adding changes for public API of the crate is in my TODO list as a next step. First thing I wanted to get feature parity with go client. Ergonomics should also be a priority.