metrics: convert processLogs callback into async-await

[trivikr]

Similar to #3794

Docs: https://googleapis.dev/nodejs/storage/5.20.5/File.html#download-examples

//-
// If the callback is omitted, we'll return a Promise.
//-
file.download().then(function(data) {
  const contents = data[0];
});

Docs: https://googleapis.dev/nodejs/storage/5.20.5/File.html#save-examples

//-
// If the callback is omitted, we'll return a Promise.
//-
file.save(contents).then(function() {});