Improve service filtering

[asimpleidea]

as of now omitting --metadata option makes the reader fatal as it does need a filter to do its job.

This may be improved by following the operator's convention:

• --metadata=*/* = I don't care what metadata they have, just pull everything

  • This puts responsibility on the adaptor, who is now in charge of filtering the resources on its own, as it will receive resources with potentially lots of metadata.
  • An Info/Warn level log may be displayed to notify user about potentially retrieving lots of services this way

• omitting --metadata = same behavior as above

• --metadata=this,cnwan.io/that = I only want resources that have key "this" AND "cnwan.io/that"

  • service with this=one 👎
  • service with this=two;cnwan.io/that=three 👍

• --metadata=cnwan.io/* = I only want resources that have key with prefix "cnwan.io", regardless of the key's name

  • service with watch=enabled 👎
  • service with cnwan.io/watch=enabled;cnwan.io/traffic-profile=standard 👍

• --metadata=*/this = I only want resources that have key name "this", regardless of its prefix

  • service with this=that 👎
  • service with cnwan.io/this=whatever 👍
  • service with dunno.lol/this=yes 👍