Simplify SelfCheck API

[alpeb]

Fixes #5575

Now that only viz makes use of the SelfCheck api, merged the healthcheck.proto into viz.proto.

Also removed the "checkRPC" functionality that was used for handling multiple API responses and was only used by SelfCheck, because the extra complexity was not granted. Revert to use the plain vanilla "check" by just concatenating error responses.

Success Output

$ bin/linkerd viz check
...
linkerd-viz
-----------
...
√ viz extension self-check

Failure Examples

Failure when viz fails to connect to the k8s api:

$ bin/linkerd viz check
...
linkerd-viz
-----------
...
× viz extension self-check
    Error calling the Kubernetes API: someerror
    see https://linkerd.io/checks/#l5d-api-control-api for hints

Status check results are ×

Failure when viz fails to connect to Prometheus:

$ bin/linkerd viz check
...
linkerd-viz
-----------
...
× viz extension self-check
    Error calling Prometheus from the control plane: someerror
    see https://linkerd.io/checks/#l5d-api-control-api for hints

Status check results are ×

Failure when viz fails to connect to both the k8s api and Prometheus:

$ bin/linkerd viz check
...
linkerd-viz
-----------
...
× viz extension self-check
    Error calling the Kubernetes API: someerror
    Error calling Prometheus from the control plane: someerror
    see https://linkerd.io/checks/#l5d-api-control-api for hints

Status check results are ×

[dadjeibaah]

This LGTM. I left one tiny TIOLI. 📦 🚢

[dadjeibaah]

tiny TIOL but I like to leave a little bit of whitespace around code block operations. I find that it helps in code readability:

diff --git a/viz/pkg/healthcheck/healthcheck.go b/viz/pkg/healthcheck/healthcheck.go
index e6ae2f44..fcf6b7e4 100644
--- a/viz/pkg/healthcheck/healthcheck.go
+++ b/viz/pkg/healthcheck/healthcheck.go
@@ -185,9 +185,11 @@ func (hc *HealthChecker) VizCategory() healthcheck.Category {
                                if err != nil {
                                        return err
                                }
+
                                if len(results.GetResults()) == 0 {
                                        return errors.New("No results returned")
                                }
+
                                errs := []string{}
                                for _, res := range results.GetResults() {
                                        if res.GetStatus() != pb.CheckStatus_OK {
@@ -197,6 +199,7 @@ func (hc *HealthChecker) VizCategory() healthcheck.Category {
                                if len(errs) == 0 {
                                        return nil
                                }
+
                                errsStr := strings.Join(errs, "\n    ")
                                return errors.New(errsStr)
                        }),

[alpeb]

Will do 👍

[adleong]

😻