Add experimental-field-mask flags

[AwesomePatrol]

Follow-up to change in cilium exposing a new experimental field in GetFlowsRequests: cilium/cilium#23198

Usage example with custom field mask and JSON output format:

% hubble observe --experimental-field-mask time,verdict --output jsonpb

Usage example with default field mask and default output format:

% hubble observe --experimental-use-default-field-masks

Example of providing invalid fieldmask:

% hubble observe --experimental-field-mask time,verdict,invalid --output jsonpb
failed to construct field mask: proto: invalid path "invalid" for message "flow.Flow"

Example of providing custom field mask for non-JSON output format:

% hubble observe --experimental-field-mask time,verdict
compact output format is not compatible with custom field mask

[chancez]

We can skip splitting inside getFlowsRequest if we switch to a StringSliceVar flag instead.

[kaworu]

@chancez I'm hesitant to tag an experimental feature as release-note/major, so switching to release-note/minor instead. It won't have effect until the next release anyway, so please reply if you feel strongly about this.

[kaworu]

CI is unhappy about the observe help output test, please update cmd/observe_help.txt accordingly.

go run ./main.go observe --help > cmd/observe_help.txt
git add -p cmd/observe_help.txt

(avoid staging the hunk about the --config flag, keeping the %s template).

[kaworu]

Thanks for the test fix @AwesomePatrol, one more thing:

Using the experimental flag (e.g. hubble observe --experimental-field-mask verdict) works well in combination with --output json, less so with other output formats:

% ./hubble observe --last 1 --experimental-field-mask verdict --output compact
N/A:  (unknown) <>  (unknown) UNKNOWN FORWARDED ()
N/A:  (unknown) <>  (unknown) UNKNOWN FORWARDED ()
% ./hubble observe --last 1 --experimental-field-mask verdict --output dict
  TIMESTAMP: N/A
     SOURCE:
DESTINATION:
       TYPE: UNKNOWN
    VERDICT: FORWARDED
    SUMMARY:
------------
  TIMESTAMP: N/A
     SOURCE:
DESTINATION:
       TYPE: UNKNOWN
    VERDICT: FORWARDED
    SUMMARY:
% ./hubble observe --last 1 --experimental-field-mask verdict --output tab
TIMESTAMP   SOURCE   DESTINATION   TYPE      VERDICT     SUMMARY
N/A                                UNKNOWN   FORWARDED
N/A                                UNKNOWN   FORWARDED
% ./hubble observe --last 1 --experimental-field-mask verdict --output json
{"flow":{"verdict":"FORWARDED"}}
{"flow":{"verdict":"FORWARDED"}}

I think we should

1. disallow --experimental-field-mask when the output format is not JSON.
2. For compact and dict/tab consider hardcoding a mask requesting only the fields used by the output format (see WriteProtoFlow).

[AwesomePatrol]

I added another flag --experimental-use-default-field-masks that specifies if we should use the new experimental field when output format other than JSON is specified. It is disabled by default. Given backwards compatibility from protobuf it is likely safe to default to true.

I also added a test to verify that all fields needed by output formats are part of the default field mask (which currently captures a bit more data than necessary).

[kaworu]

Thanks for the quick update @AwesomePatrol 🎉

Couple of nitpicking comments, other than that LGTM.