create a working example of vector internal metrics monitoring

[jdrouet]

This PR follows a question asked on discord.

Signed-off-by: Jérémie Drouet [email protected]

[lucperkins]

I'd recommend adding something to the docs that lets a user explore the produced metrics. Perhaps you could link to something like this URL, which displays a graph for the vector_events_in_total{component_name="blackhole"} metric across a 5-minute time range:

http://localhost:9090/graph?g0.expr=vector_events_in_total%7Bcomponent_name%3D%22blackhole%22%7D&g0.tab=0&g0.range_input=5m