Add network interface stats

We do not have to collect these often, so for now set the collection
interval to 120s (even though the Stackdriver exporter is still set to
export every 60s).

test/e2e-install.sh

@@ -47,13 +47,16 @@ function install-npd() {
 
   readonly workdir=$(mktemp -d)
   tar -xf "${TARBALL}" --directory "${workdir}"
-  
+
   echo "Preparing NPD binary directory."
   mkdir -p "${BIN_DIR}"
   mount --bind "${BIN_DIR}" "${BIN_DIR}"
   # Below remount is to work around COS's noexec mount on /home.
   mount -o remount,exec "${BIN_DIR}"
 
+  echo "Stopping NPD"
+  systemctl stop node-problem-detector.service || true
+
   echo "Installing NPD binary."
   cp "${workdir}"/bin/node-problem-detector "${BIN_DIR}"
 
@@ -75,7 +78,6 @@ function install-npd() {
   # Start systemd service.
   echo "Starting NPD systemd service."
   systemctl daemon-reload
-  systemctl stop node-problem-detector.service || true
   systemctl start node-problem-detector.service
 }
 
@@ -97,4 +99,4 @@ done
 shift "$((OPTIND-1))"
 
 
-main "${@}"
+main "${@}"