No way to get the etcd revision from puts and transaction transaction puts

We want to check the whole etcd hasn't changed, for reasons... Would it be cool to return them instead of None, something like the following:

```
diff --git a/etcd3/client.py b/etcd3/client.py
index 662d125..55a85df 100644
--- a/etcd3/client.py
+++ b/etcd3/client.py
@@ -346,12 +346,13 @@ class Etcd3Client(object):
:type lease: either :class:`.Lease`, or int (ID of lease)
"""
put_request = self._build_put_request(key, value, lease=lease)
- self.kvstub.Put(
+ meta_data = self.kvstub.Put(
put_request,
self.timeout,
credentials=self.call_credentials,
metadata=self.metadata
)
+ return (True, meta_data)

@_handle_errors
def replace(self, key, initial_value, new_value):
@@ -644,7 +645,7 @@ class Etcd3Client(object):
for response in txn_response.responses:
response_type = response.WhichOneof('response')
if response_type == 'response_put':
- responses.append(None)
+ responses.append(response)

elif response_type == 'response_range':
range_kvs = []

```

Start in etcd3/client.py at the put method and the transaction response loop, then inspect how kvstub.Put metadata and response_put values are currently handled. Done means puts and transaction puts expose the etcd revision metadata instead of returning None, while preserving the existing response behavior.

Written by the indexing model from the issue text.