Add fix for re-registration when Consul K8s rolling restarts agent nodes

[andrewstucki]

Changes proposed in this PR:

Currently K8s makes agent nodes ephemeral, when an agent node is upgraded it persists no local state because we don't mount any sort of persistent storage to the agent node containers. This means that we need to properly re-register our services when the agents restart. We currently have some re-registration logic within our registration code when it's detected that a service is non-existent, but it looks like our error casting code was broken due to having an extra pointer.

Unfortunately, this code path is quite tricky to hit without standing up an additional Consul cluster, and if we want to actually test the reproduction case, it'd require orchestrating an agent node flapping while dropping all of it's state. So, I figure manual testing might be better -- we have a mock Consul server that's able to unit test all the other code paths already.

How I've tested this PR:

In the process of ensuring that the deployments get re-registered when a rolling restart happens by performing one manually.

Validated via the learn tutorial with a custom image build and triggering a restart of the consul agent nodes. Here are the logs from the deployment (notice the successfully registered agent service message third from the bottom):

{"timestamp":"2022-06-16 13:54:59.229","thread":"19","level":"warning","name":"config","source":"./source/common/config/grpc_stream.h:160","message":"DeltaAggregatedResources gRPC config stream closed: 13, "}
{"@caller":"/go/pkg/mod/github.com/hashicorp/consul/api@v1.12.1-0.20220111183205-dcf1cd485363/watch/plan.go:95","@level":"error","@message":"Watch errored","@module":"consul-api-gateway-exec.cert-manager.watch","@timestamp":"2022-06-16T13:54:59.232953Z","error":"Unexpected response code: 500 (rpc error making call: EOF)","retry":5000000000,"type":"connect_roots"}
{"@caller":"/go/pkg/mod/github.com/hashicorp/consul/api@v1.12.1-0.20220111183205-dcf1cd485363/watch/plan.go:95","@level":"error","@message":"Watch errored","@module":"consul-api-gateway-exec.cert-manager.watch","@timestamp":"2022-06-16T13:55:00.240727Z","error":"Get \"https://172.18.0.2:8501/v1/agent/connect/ca/leaf/example-gateway\": dial tcp 172.18.0.2:8501: connect: connection refused","retry":5000000000,"type":"connect_leaf"}
{"@caller":"/build/internal/consul/registration.go:110","@level":"error","@message":"error fetching service","@module":"consul-api-gateway-exec.service-registry","@timestamp":"2022-06-16T13:55:02.724288Z","error":"Get \"https://172.18.0.2:8501/v1/agent/service/9eb6fa1e-43ee-474c-b55f-fc81e794ce81\": dial tcp 172.18.0.2:8501: connect: connection refused"}
{"@caller":"/go/pkg/mod/github.com/hashicorp/consul/api@v1.12.1-0.20220111183205-dcf1cd485363/watch/plan.go:95","@level":"error","@message":"Watch errored","@module":"consul-api-gateway-exec.cert-manager.watch","@timestamp":"2022-06-16T13:55:04.234505Z","error":"Get \"https://172.18.0.2:8501/v1/agent/connect/ca/roots\": dial tcp 172.18.0.2:8501: connect: connection refused","retry":20000000000,"type":"connect_roots"}
{"@caller":"/go/pkg/mod/github.com/hashicorp/consul/api@v1.12.1-0.20220111183205-dcf1cd485363/watch/plan.go:95","@level":"error","@message":"Watch errored","@module":"consul-api-gateway-exec.cert-manager.watch","@timestamp":"2022-06-16T13:55:05.242260Z","error":"Get \"https://172.18.0.2:8501/v1/agent/connect/ca/leaf/example-gateway\": dial tcp 172.18.0.2:8501: connect: connection refused","retry":20000000000,"type":"connect_leaf"}
{"@caller":"/build/internal/consul/registration.go:103","@level":"info","@message":"successfully registered agent service","@module":"consul-api-gateway-exec.service-registry","@timestamp":"2022-06-16T13:55:32.787988Z"}
{"timestamp":"2022-06-16 13:55:32.989","thread":"19","level":"info","name":"upstream","source":"source/common/upstream/cds_api_helper.cc:30","message":"cds: add 0 cluster(s), remove 0 cluster(s)"}
{"timestamp":"2022-06-16 13:55:32.990","thread":"19","level":"info","name":"upstream","source":"source/common/upstream/cds_api_helper.cc:67","message":"cds: added/updated 0 cluster(s), skipped 0 unmodified cluster(s)"}

Checklist:

• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[sarahalsmiller]

Looks good to me.