etcdserver: Replace value contents with value_size in request took too long warning

[jpbetz]

This removes the logging of values for "request took too long warnings" that was introduced in etcd 3.2. We shouldn't log values. Value fields are likely contain sensitive data, make the logs difficult to scan, and have the potential to "flood" the logs exhausting disk space and/or starving the system disk IO.

This replaces all occurrences of value fields with a value_size field. Fortunately there are only two occurrences: (1) put requests and (2) txns that do a value compare.

Example output:

txn:

BEFORE:
2018-06-07 16:54:52.146394 W | etcdserver: request "header:<ID:7587830746628055043 > txn:<compare:<target:VALUE key:\"a\" value:\"1\" > success:<request_put:<key:\"b\" value:\"foo\" > > failure:<request_put:<key:\"c\" value:\"goo\\\"\" > > > " took too long (106.821µs) to execute

AFTER:
2018-06-07 16:37:42.945095 W | etcdserver: request "header:<ID:7587830746365689091 > txn:<compare:<target:VALUE key:\"a\" value_size:1 > success:<request_put:<key:\"b\" value_size:3 >> failure:<request_put:<key:\"c\" value_size:4 >>>" took too long (71.196µs) to execute

put:

BEFORE:
2018-06-07 16:54:58.694712 W | etcdserver: request "header:<ID:7587830746628055044 > put:<key:\"a\" value:\"this is a test\" > " took too long (106.008µs) to execute

AFTER:
2018-06-07 16:37:58.844290 W | etcdserver: request "header:<ID:7587830746365689092 > put:<key:\"a\" value_size:14 >" took too long (59.798µs) to execute

In order to preserve proto encoding of []byte fields, this introduces some private structs mirroring the request proto structs, but with value replaced with value_size.

cc @gyuho @wenjiaswe @mml

[gyuho]

lgtm thanks!

[wenjiaswe]

LGTM