OPA /health?plugins API panic's with envoy plugin

[patrick-east]

Using the latest OPA if you configure a health probe with the envoy plugin https://github.com/open-policy-agent/opa-istio-plugin you will get a panic:

{15:48} ✘ ~ ❯ k logs example-app-79586cfcdb-r9gvj opa
{"addrs":["localhost:8181"],"diagnostic-addrs":["0.0.0.0:8282"],"insecure_addr":"","level":"info","msg":"Initializing server.","time":"2020-05-12T22:47:44Z"}
{"level":"info","msg":"Starting decision logger.","plugin":"decision_logs","time":"2020-05-12T22:47:44Z"}
{"addr":":9191","dry-run":false,"enable-reflection":false,"level":"info","msg":"Starting gRPC server.","path":"","query":"data.envoy.authz.allow","time":"2020-05-12T22:47:44Z"}
{"client_addr":"172.17.0.1:38724","level":"info","msg":"Received request.","req_body":"","req_id":1,"req_method":"GET","req_params":{"plugins":[""]},"req_path":"/health","time":"2020-05-12T22:47:49Z"}
2020/05/12 22:47:49 http: panic serving 172.17.0.1:38724: runtime error: invalid memory address or nil pointer dereference
goroutine 6 [running]:
net/http.(*conn).serve.func1(0xc0002a2140)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:1772 +0x139
panic(0xc85c00, 0x14fcdc0)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/runtime/panic.go:975 +0x3e3
github.com/open-policy-agent/opa/server.(*Server).unversionedGetHealth(0xc0001e11e0, 0xee6580, 0xc00000c4c0, 0xc0002d4600)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/server/server.go:1011 +0x19d
net/http.HandlerFunc.ServeHTTP(0xc000235910, 0xee6580, 0xc00000c4c0, 0xc0002d4600)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:2012 +0x44
github.com/open-policy-agent/opa/internal/prometheus.(*Provider).InstrumentHandler.func1(0x7f2eda291108, 0xc0000791d0, 0xc0002d4600)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/internal/prometheus/prometheus.go:78 +0xfc
net/http.HandlerFunc.ServeHTTP(0xc00023d440, 0x7f2eda291108, 0xc0000791d0, 0xc0002d4600)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:2012 +0x44
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerDuration.func1(0xee6740, 0xc00007c440, 0xc0002d4600)
	/Users/patrick/projects/go/pkg/mod/github.com/prometheus/[email protected]/prometheus/promhttp/instrument_server.go:68 +0x11c
net/http.HandlerFunc.ServeHTTP(0xc00023b860, 0xee6740, 0xc00007c440, 0xc0002d4600)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:2012 +0x44
github.com/gorilla/mux.(*Router).ServeHTTP(0xc0001b6180, 0xee6740, 0xc00007c440, 0xc0002d4300)
	/Users/patrick/projects/go/pkg/mod/github.com/gorilla/[email protected]/mux.go:212 +0xe2
github.com/open-policy-agent/opa/runtime.(*LoggingHandler).ServeHTTP(0xc0003051a0, 0xee7bc0, 0xc0002e2000, 0xc0002d4300)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/runtime/logging.go:83 +0x153
net/http.serverHandler.ServeHTTP(0xc0001f61c0, 0xee7bc0, 0xc0002e2000, 0xc0002d4300)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:2807 +0xa3
net/http.(*conn).serve(0xc0002a2140, 0xeea940, 0xc00007c2c0)
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:1895 +0x86c
created by net/http.(*Server).Serve
	/usr/local/Cellar/go/1.14.2_1/libexec/src/net/http/server.go:2933 +0x35c

The issue appears to be that the plugin doesn't have a status set, which is OK, but the server tries to dereference the status pointer and barfs.