examples(latency): Add example to calculate client and server latency.

[danielmai]

This is for CLOUD-260.

This adds an example to get the client-side and the server-side latency of a query.

The important bit is the following snippet:

dgraph-js-http/examples/latency/index-async-await.js

Lines 112 to 115 in 88ffe94

	console.time('Query client latency');
	const res = await dgraphClient.newTxn().queryWithVars(query, vars);
	console.timeEnd('Query client latency');
	console.log("Query server latency:", JSON.stringify(res.extensions.server_latency));

Client-side latency is calculated using console.time and console.timeEnd around a query call to capture the client-side latency.

Server-side latency is calculated by Dgraph itself. This example prints it out from the query response res.extensions.server_latency.

---

Example command and output:

$ node index-async-await.js --addr https://$MY_DGRAPH_CLOUD_ENDPOINT --api-key $MY_API_KEY --drop-all
Created person named "Alice" with uid = 0xbd

All created nodes (map from blank node names to uids):
blank-0 => 0xbd
dg.3043386991.125 => 0xbe
dg.3043386991.126 => 0xbf
dg.3043386991.127 => 0xc0

Query: query all($a: string) { all(func: eq(name, $a)) { uid name age married loc dob friend { name age } school { name } } } Vars: { '$a': 'Alice' }
Query client latency: 39.371ms
Query server latency: {"parsing_ns":299313,"processing_ns":1606551,"encoding_ns":361067,"assign_timestamp_ns":1389885,"total_ns":3770262}
Number of people named "Alice": 1
{
  uid: '0xbd',
  name: 'Alice',
  age: 26,
  married: true,
  loc: { type: 'Point', coordinates: [ 1.1, 2 ] },
  dob: '1980-02-02T07:00:00Z',
  friend: [ { name: 'Bob', age: 24 }, { name: 'Charlie', age: 29 } ],
  school: [ { name: 'Crown Public School' } ]
}

DONE!

The following flags are available in this example:

• --addr: The Dgraph address. Can be a Dgraph Cloud endpoint following the instructions: https://github.com/dgraph-io/dgraph-js-http#create-a-client-for-dgraph-cloud-endpoint. Defaults to http://localhost:8080.
• --api-key: The Dgraph Cloud API Key. Required if the --addr is a Dgraph Cloud endpoint.
• --drop-all: Call DropAll on Dgraph before loading the data for this example.

This change is