Make verbosity level mandatory with telemetry opt

[pscott]

• Closes CLI: Telemetry URL & Verbosity Flags #4372

As mentioned in #4372 , this PR makes the verbosity level mandatory in the --telemetry-urls option, instead of defaulting to 0.

There are a couple of points I would like to clarify:

1. I'm not sure this is considering breaking the API? Or should I flag it as B2-breaksapi?
2. I don't think this PR changes anything to Polkadot but I'm not sure?
3. I haven't managed to find the place where we check that the VERBOSITY_LEVEL is not bigger than the maximum verbosity_level. Maybe there's no point in doing so, but it might be better practice to let the end user know that when he specifies a verbosity-level bigger than 9, it doesn't actually exist?
4. As for verbosity level, I've found that the constant SUBSTRATE_DEBUG is declared here

   substrate/client/telemetry/src/lib.rs

   Line 108 in db1ab7d

   pub const SUBSTRATE_DEBUG: &str = "9";

   but I have not managed to find where it is USED in the code. Is it just here for export purposes?

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[pscott]

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[pscott]

Currently waiting on #5069 to be reviewed (and maybe) merged, and then will be able to address comments and get ready for review. :D

Review has been addressed but is not showing up as addressed