feat: Reworked MutateRows to use the data client (#1290)

**Changes Made:**

- Rerouted MutateRows to use the data client's `Table.bulk_mutate_rows`
function.
- Reworked error handling in `Table.mutate_rows`. Since we must return a
list of `Status` objects, we process the list of errors obtained from
the `_MutateRowsOperation` back into a corresponding list of error
statuses.
- Reworked retries in `Table.mutate_rows` as follows:
  - `retry.deadline` -> `operation_timeout`
  - `timeout`/`mutation_timeout` -> `attempt_timeout`
- A null retry deadline now defaults to the table default mutate rows
operation timeout instead of being defined as no timeout. The docstring
has been updated to reflect that. A retry deadline of 0.0 will still be
a no-retry option, and is implemented by setting the list of retriable
errors to the empty list, and the operation timeout to the default
mutate rows operation timeout.
- A `timestamp` of `None` in `set_cell`, which is the default, will now
generate a client-side timestamp representing the current time, rather
than defaulting to using the server-side timestamp. The docstrings for
this have been updated to reflect this.

Author: gkevinzheng

google/cloud/bigtable/data/exceptions.py

@@ -141,6 +141,8 @@ def __repr__(self):
         return f"{self.__class__.__name__}({message!r}, {self.exceptions!r})"
 
 
+# TODO: When working on mutations batcher, rework exception handling to guarantee that
+# MutationsExceptionGroup only stores FailedMutationEntryErrors. 
 class MutationsExceptionGroup(_BigtableExceptionGroup):
     """
     Represents one or more exceptions that occur during a bulk mutation operation

google/cloud/bigtable/row.py

@@ -153,15 +153,17 @@ def _set_cell(self, column_family_id, column, value, timestamp=None, state=None)
                       integer (8 bytes).
 
         :type timestamp: :class:`datetime.datetime`
-        :param timestamp: (Optional) The timestamp of the operation.
+        :param timestamp: (Optional) The timestamp of the operation. If a
+                         timestamp is not provided, the current system time
+                         will be used.
 
         :type state: bool
         :param state: (Optional) The state that is passed along to
                       :meth:`_get_mutations`.
         """
-        if timestamp is None:
-            # Use current Bigtable server time.
-            timestamp_micros = mutations._SERVER_SIDE_TIMESTAMP
+        if timestamp is None or timestamp == mutations._SERVER_SIDE_TIMESTAMP:
+            # Preserve special-case values (client side timestamp generation or server side timestamp)
+            timestamp_micros = timestamp
         else:
             timestamp_micros = _microseconds_from_datetime(timestamp)
             # Truncate to millisecond granularity.
@@ -351,7 +353,9 @@ def set_cell(self, column_family_id, column, value, timestamp=None):
                       integer (8 bytes).
 
         :type timestamp: :class:`datetime.datetime`
-        :param timestamp: (Optional) The timestamp of the operation.
+        :param timestamp: (Optional) The timestamp of the operation. If a
+                         timestamp is not provided, the current system time
+                         will be used.
         """
         self._set_cell(column_family_id, column, value, timestamp=timestamp, state=None)
 
@@ -651,7 +655,9 @@ def set_cell(self, column_family_id, column, value, timestamp=None, state=True):
                       integer (8 bytes).
 
         :type timestamp: :class:`datetime.datetime`
-        :param timestamp: (Optional) The timestamp of the operation.
+        :param timestamp: (Optional) The timestamp of the operation. If a
+                         timestamp is not provided, the current system time
+                         will be used.
 
         :type state: bool
         :param state: (Optional) The state that the mutation should be