fix: do not load google-gax at client startup

[schmidt-sebastian]

This improves performance for functions users that only read data from the provided DocumentSnapshot, but do not otherwise access Firestore.

Test file (from @inlined): https://gist.github.com/schmidt-sebastian/f42c30d75785fba436746f894948ecf4

Extra dependencies loaded before PR:

@google-cloud
@grpc
@protobufjs
abort-controller
arrify
base64-js
bignumber.js
buffer-equal-constant-time
duplexify
ecdsa-sig-formatter
end-of-stream
event-target-shim
extend
fast-deep-equal
firebase-admin
functional-red-black-tree
gaxios
gcp-metadata
google-auth-library
google-gax
gtoken
inherits
is-stream
is-stream-ended
json-bigint
jwa
jws
lodash.camelcase
long
lru-cache
node-fetch
node-forge
object-hash
once
protobufjs
readable-stream
retry-request
safe-buffer
stream-shift
util-deprecate
wrappy
yallist

After:

@google-cloud
fast-deep-equal
firebase-admin
node-forge

Cold start time from firebase-functions:

Invocation	Branch (ms)	Master (ms)
0	1142	3035
1	1034	965
2	743	1080
3	728	728
4	662	738
5	675	851
6	644	791
7	824	763
8	970	716
9	1438	734
Average	886	1040
Worst Case	1438	3035

Code:

const functions = require('firebase-functions');

exports.myFunction = functions.firestore
  .document('foo/{docId}')
  .onCreate((change, context) => { 
	  console.info('Executed for document ' + change.id);
	  setTimeout(() => process.exit(0), 1);
});   

[thebrianchen]

If require() automatically caches, the modules it loads, why did we cache the modules on this class to begin with?

[schmidt-sebastian]

Coding is a learning opportunity for all of us.

[thebrianchen]

nit: flip undefined to the other side

[schmidt-sebastian]

Done

[inlined]

I ran my benchmark agains this branch. The benchmark uses 60 shards which will each try 20 times to purge the require cache and load both the firebase-functions SDK and execute the boilerplate that happens in the cold start for a Firestore function. Here are my results:

stat.	vanilla	optimized	savings
mean	335	253	24%
p50	273	224	18%
p90	513	485	5%
p95	911	589	35%
p99	1498	1150	23%