Minor refactorings (#368)

* Split vkubelet funcitons into separate files.

* Minor re-org for cmd/census*

* refactor run loop

cmd/census_tracing.go

@@ -0,0 +1,69 @@
+package cmd
+
+import (
+	"context"
+	"net/http"
+	"os"
+
+	"github.com/cpuguy83/strongerrors"
+	"github.com/pkg/errors"
+	"github.com/virtual-kubelet/virtual-kubelet/log"
+	"go.opencensus.io/trace"
+	"go.opencensus.io/zpages"
+)
+
+var (
+	tracingExporters = make(map[string]TracingExporterInitFunc)
+
+	reservedTagNames = map[string]bool{
+		"operatingSystem": true,
+		"provider":        true,
+		"nodeName":        true,
+	}
+)
+
+// TracingExporterOptions is used to pass options to the configured tracer
+type TracingExporterOptions struct {
+	Tags        map[string]string
+	ServiceName string
+}
+
+// TracingExporterInitFunc is the function that is called to initialize an exporter.
+// This is used when registering an exporter and called when a user specifed they want to use the exporter.
+type TracingExporterInitFunc func(TracingExporterOptions) (trace.Exporter, error)
+
+// RegisterTracingExporter registers a tracing exporter.
+// For a user to select an exporter, it must be registered here.
+func RegisterTracingExporter(name string, f TracingExporterInitFunc) {
+	tracingExporters[name] = f
+}
+
+// GetTracingExporter gets the specified tracing exporter passing in the options to the exporter init function.
+// For an exporter to be availbale here it must be registered with `RegisterTracingExporter`.
+func GetTracingExporter(name string, opts TracingExporterOptions) (trace.Exporter, error) {
+	f, ok := tracingExporters[name]
+	if !ok {
+		return nil, strongerrors.NotFound(errors.Errorf("tracing exporter %q not found", name))
+	}
+	return f(opts)
+}
+
+// AvailableTraceExporters gets the list of registered exporters
+func AvailableTraceExporters() []string {
+	out := make([]string, 0, len(tracingExporters))
+	for k := range tracingExporters {
+		out = append(out, k)
+	}
+	return out
+}
+
+func setupZpages() {
+	ctx := context.TODO()
+	p := os.Getenv("ZPAGES_PORT")
+	if p == "" {
+		log.G(ctx).Error("Missing ZPAGES_PORT env var, cannot setup zpages endpoint")
+	}
+	mux := http.NewServeMux()
+	zpages.Handle(mux, "/debug")
+	http.ListenAndServe(p, mux)
+}