Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/contrib.go.opencensus.io/exporter/ocagent/options.go

@@ -14,7 +14,11 @@
 
 package ocagent
 
-import "time"
+import (
+	"time"
+
+	"google.golang.org/grpc/credentials"
+)
 
 const (
 	DefaultAgentPort uint16 = 55678
@@ -76,3 +80,49 @@ func (rp reconnectionPeriod) withExporter(e *Exporter) {
 func WithReconnectionPeriod(rp time.Duration) ExporterOption {
 	return reconnectionPeriod(rp)
 }
+
+type compressorSetter string
+
+func (c compressorSetter) withExporter(e *Exporter) {
+	e.compressor = string(c)
+}
+
+// UseCompressor will set the compressor for the gRPC client to use when sending requests.
+// It is the responsibility of the caller to ensure that the compressor set has been registered
+// with google.golang.org/grpc/encoding. This can be done by encoding.RegisterCompressor. Some
+// compressors auto-register on import, such as gzip, which can be registered by calling
+// `import _ "google.golang.org/grpc/encoding/gzip"`
+func UseCompressor(compressorName string) ExporterOption {
+	return compressorSetter(compressorName)
+}
+
+type headerSetter map[string]string
+
+func (h headerSetter) withExporter(e *Exporter) {
+	e.headers = map[string]string(h)
+}
+
+// WithHeaders will send the provided headers when the gRPC stream connection
+// is instantiated
+func WithHeaders(headers map[string]string) ExporterOption {
+	return headerSetter(headers)
+}
+
+type clientCredentials struct {
+	credentials.TransportCredentials
+}
+
+var _ ExporterOption = (*clientCredentials)(nil)
+
+// WithTLSCredentials allows the connection to use TLS credentials
+// when talking to the server. It takes in grpc.TransportCredentials instead
+// of say a Certificate file or a tls.Certificate, because the retrieving
+// these credentials can be done in many ways e.g. plain file, in code tls.Config
+// or by certificate rotation, so it is up to the caller to decide what to use.
+func WithTLSCredentials(creds credentials.TransportCredentials) ExporterOption {
+	return &clientCredentials{TransportCredentials: creds}
+}
+
+func (cc *clientCredentials) withExporter(e *Exporter) {
+	e.clientTransportCredentials = cc.TransportCredentials
+}