feat: log REST request headers (#1497)

Note that the REST server logs metadata unconditionally, so logging the request headers is not gated on passing `-v` to Showcase. Thus, to show headers for both the gRPC and REST transports, `gapic-showcase run -v` will work.

util/genrest/goviewcreator.go

@@ -91,6 +91,7 @@ func NewView(model *gomodel.Model) (*goview.View, error) {
 			// number of path variables, etc.)
 			source.P(`  backend.StdLog.Printf("Received %%s request matching '%s': %%q", r.Method, r.URL)`, handler.URIPattern)
 			source.P(`  backend.StdLog.Printf("  urlPathParams (expect %d, have %%d): %%q", numUrlPathParams, urlPathParams)`, len(allURLVariables))
+			source.P(`  backend.StdLog.Printf("  urlRequestHeaders:\n%%s", resttools.PrettyPrintHeaders(r, "    "))`)
 			source.P("")
 			source.P("  if numUrlPathParams!=%d {", len(allURLVariables))
 			source.P(`    backend.Error(w, http.StatusBadRequest, "found unexpected number of URL variables: expected %d, have %%d: %%#v", numUrlPathParams, urlPathParams)`, len(allURLVariables))

util/genrest/resttools/headers.go

@@ -83,3 +83,18 @@ func CheckAPIClientHeader(header http.Header) error {
 	}
 	return fmt.Errorf("internal inconsistency")
 }
+
+// PrettyPrintHeaders prints all the HTTP headers in `request` in
+// lines preceded by `indentation`. Each line has one header key and a
+// quoted list of all values for that key.
+func PrettyPrintHeaders(request *http.Request, indentation string) string {
+	lines := []string{}
+	for key, values := range request.Header {
+		newLine := fmt.Sprintf("%s%s:", indentation, key)
+		for _, oneValue := range values {
+			newLine += fmt.Sprintf(" %q", oneValue)
+		}
+		lines = append(lines, newLine)
+	}
+	return strings.Join(lines, "\n")
+}