Add UDF to discover namespace where Vizier is deployed

[rosenhouse]

Summary: Adds a new px.vizier_namespace() function usable from PxL scripts to discover the namespace that the Vizier components are deployed into.

Type of change: /kind feature

Test Plan: Added some unit tests. Manually tested on a minikube.

Relevant Issues: Fixes #1304

[pixie-io-buildbot]

Can one of the admins verify this patch?

[rosenhouse]

I mentioned this in Slack too, but just to surface the issue here:

I'm naming this thing vizier_namespace throughout, but I notice there is some prior art on this value:

• kelvin has a PL_NAMESPACE environment variable flowing into a FLAGS_namespace

  pixie/src/vizier/services/agent/kelvin/kelvin_main.cc

  Lines 41 to 42 in 093d66e

  	DEFINE_string(namespace, gflags::StringFromEnv("PL_NAMESPACE", "pl"),
  	"The namespace of the Vizier instance");

• other components have a PL_POD_NAMESPACE , e.g.

  pixie/k8s/vizier/persistent_metadata/base/metadata_statefulset.yaml

  Lines 79 to 82 in 093d66e

  	- name: PL_POD_NAMESPACE
  	valueFrom:
  	fieldRef:
  	fieldPath: metadata.namespace

So perhaps I should:

1. have the env var as PL_POD_NAMESPACE or PL_NAMESPACE
2. have the fields in the metadata code be pod_namespace
3. keep the UDF as vizier_namespace() in order to make it clear that this UDF is about the vizier itself, not the http event data that it's processing.

Thoughts?

[JamesMBartlett]

Thanks for your contribution, love the idea of making this programmatic rather than hardcoding pl in scripts.

I mentioned this in Slack too, but just to surface the issue here:

I'm naming this thing vizier_namespace throughout, but I notice there is some prior art on this value:

* kelvin has a `PL_NAMESPACE` environment variable flowing into a `FLAGS_namespace` https://github.com/pixie-io/pixie/blob/093d66e251e4cdf45cf72430cdd4f64ca83b5cb1/src/vizier/services/agent/kelvin/kelvin_main.cc#L41-L42

* other components have a `PL_POD_NAMESPACE` , e.g. https://github.com/pixie-io/pixie/blob/093d66e251e4cdf45cf72430cdd4f64ca83b5cb1/k8s/vizier/persistent_metadata/base/metadata_statefulset.yaml#L79-L82

So perhaps I should:

1. have the env var as `PL_POD_NAMESPACE` or `PL_NAMESPACE`

I would call the env var PL_POD_NAMESPACE

2. have the fields in the metadata code be `pod_namespace`

I think leaving these as vizier_namespace is good.

3. keep the UDF as `vizier_namespace()` in order to make it clear that this UDF is about the vizier itself, not the http event data that it's processing.

yeah I think vizier_namespace is good here

[JamesMBartlett]

@pixie-io-buildbot test this please