Debug operator

[mattnibs]

This commit introduces the debug operator- an operator intending to help debugging complex queries. When running the debug operator using zq or zed all encounterd values at the point of the operator are written to stderr.

Closes #5181

[nwt]

@philrz and I talked about extending the Zed syntax so you can specify a channel other than "debug". We both think that's a good idea since Zui will eventually be able to display channels separately but we can add it later.

[mattnibs]

@philrz and I talked about extending the Zed syntax so you can specify a channel other than "debug". We both think that's a good idea since Zui will eventually be able to display channels separately but we can add it later.

Is this different than using the output operator?

[nwt]

@philrz and I talked about extending the Zed syntax so you can specify a channel other than "debug". We both think that's a good idea since Zui will eventually be able to display channels separately but we can add it later.

Is this different than using the output operator?

Yes, in two regards. First, it includes an expression (e.g., debug some_channel some.path). And second, it doesn't have output's "read to EOS" behavior.