tfsdklog: Consolidated multiple invalid log level messages and added missing newline (#35)

Reference: #32

This is related to SDK (or special implementations of testing framework) code which needs to execute `tfsdklog.RegisterTestSink()`, generally via the `tf5server.WithLoggingSink()`/`tf6server.WithLoggingSink()` functions.

Previously in terraform-provider-kubernetes:

```console
$ TF_LOG=trac go test -v -tags acceptance -run '^TestKubernetesManifest_ConfigMap$' ./manifest/test/acceptance
2022/03/04 16:51:21 Testing against Kubernetes API version: v1.22.3
=== RUN   TestKubernetesManifest_ConfigMap
[WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF][WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF][WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF][WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF][WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF][WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF]...
```

After updating dependency:

```console
$ go mod edit -replace=github.com/hashicorp/terraform-plugin-log=/Users/bflad/src/github.com/hashicorp/terraform-plugin-log
$ go mod tidy
$ go mod vendor
$ TF_LOG=trac go test -v -tags acceptance -run '^TestKubernetesManifest_ConfigMap$' ./manifest/test/acceptance
2022/03/04 16:52:49 Testing against Kubernetes API version: v1.22.3
=== RUN   TestKubernetesManifest_ConfigMap
[WARN] Invalid log level: "TRAC". Defaulting to level: OFF. Valid levels are: [TRACE DEBUG INFO WARN ERROR OFF]
... other test output ...
```

Author: bflad

.changelog/35.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+tfsdklog: Consolidated multiple invalid log level messages and added missing newline
+```

tfsdklog/sink.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"strings"
+	"sync"
 	"syscall"
 
 	"github.com/hashicorp/go-hclog"
@@ -53,6 +54,9 @@ const (
 // loggers.
 var ValidLevels = []string{"TRACE", "DEBUG", "INFO", "WARN", "ERROR", "OFF"}
 
+// Only show invalid log level message once across any number of level lookups.
+var invalidLogLevelMessage sync.Once
+
 func getSink(ctx context.Context) hclog.Logger {
 	logger := ctx.Value(logging.SinkKey)
 	if logger == nil {
@@ -120,8 +124,14 @@ func newSink(t testing.T) hclog.Logger {
 	} else if isValidLogLevel(envLevel) {
 		logLevel = hclog.LevelFromString(envLevel)
 	} else {
-		fmt.Fprintf(os.Stderr, "[WARN] Invalid log level: %q. Defaulting to level: OFF. Valid levels are: %+v",
-			envLevel, ValidLevels)
+		invalidLogLevelMessage.Do(func() {
+			fmt.Fprintf(
+				os.Stderr,
+				"[WARN] Invalid log level: %q. Defaulting to level: OFF. Valid levels are: %+v\n",
+				envLevel,
+				ValidLevels,
+			)
+		})
 	}
 
 	return hclog.New(&hclog.LoggerOptions{