Add :metrics option to capture headers

[elersong]

Notes

Current limitations of the JS driver prevent easy access to usage metrics provided in the response headers from the DB. This PR adds a :metrics option to allow programmatic access to those metrics.

How to test

yarn test

[faunaee]

Nice! Many JS driver users will appreciate this feature. I made a few suggestions.

[ptpaterson]

Can we add another method to Client as a shorthand for setting the withMetrics option? This would harmonize it with the JVM drivers. Something like:

/**
 * Executes a query via the FaunaDB Query API.  
 * See the [docs](https://app.fauna.com/documentation/reference/queryapi),
 * and the query functions in this documentation.
 * @param expression {module:query~ExprArg}
 *   The query to execute. Created from {@link module:query} functions.
 * @param {?Object} options
 *   Object that configures the current query, overriding FaunaDB client options.
 * @param {?string} options.secret FaunaDB secret (see [Reference Documentation](https://app.fauna.com/documentation/intro/security))
 * @return {external:Promise<Object>} An object containing the FaunaDB response object and the list of query metrics incurred by the request.
 */
Client.prototype.queryWithMetrics = function(expression, options) {
  options = Object.assign({}, options, { withMetrics: true })
  return this._execute('POST', '', query.wrap(expression), null, options)
}

Addressed

[faunaee]

A couple of minor suggestions.

Also, we should include a test and an update to README.md describing this feature.

addressed