Fix @tag_args being off-by-one (ahead)
Example:
```
argspec.args=[
'self',
'room_id'
]
args=(
<synapse.storage.databases.main.DataStore object at 0x10d0b8d00>,
'!HBehERstyQBxyJDLfR:my.synapse.server'
)
```
---
The previous logic was also flawed and we can end up in a situation like this:
```
argspec.args=['self', 'dest', 'room_id', 'limit', 'extremities']
args=(<synapse.federation.federation_client.FederationClient object at 0x7f1651c18160>, 'hs1', '!jAEHKIubyIfuLOdfpY:hs1')
```
From this source:
```py
async def backfill(
self, dest: str, room_id: str, limit: int, extremities: Collection[str]
) -> Optional[List[EventBase]]:
```
And this usage:
```py
events = await self._federation_client.backfill(
dest, room_id, limit=limit, extremities=extremities
)
```
which would previously cause this error:
```
synapse_main | 2022-08-04 06:13:12,051 - synapse.handlers.federation - 424 - ERROR - GET-5 - Failed to backfill from hs1 because tuple index out of range
synapse_main | Traceback (most recent call last):
synapse_main | File "/usr/local/lib/python3.9/site-packages/synapse/handlers/federation.py", line 392, in try_backfill
synapse_main | await self._federation_event_handler.backfill(
synapse_main | File "/usr/local/lib/python3.9/site-packages/synapse/logging/tracing.py", line 828, in _wrapper
synapse_main | return await func(*args, **kwargs)
synapse_main | File "/usr/local/lib/python3.9/site-packages/synapse/handlers/federation_event.py", line 593, in backfill
synapse_main | events = await self._federation_client.backfill(
synapse_main | File "/usr/local/lib/python3.9/site-packages/synapse/logging/tracing.py", line 828, in _wrapper
synapse_main | return await func(*args, **kwargs)
synapse_main | File "/usr/local/lib/python3.9/site-packages/synapse/logging/tracing.py", line 827, in _wrapper
synapse_main | with wrapping_logic(func, *args, **kwargs):
synapse_main | File "/usr/local/lib/python3.9/contextlib.py", line 119, in __enter__
synapse_main | return next(self.gen)
synapse_main | File "/usr/local/lib/python3.9/site-packages/synapse/logging/tracing.py", line 922, in _wrapping_logic
synapse_main | set_attribute("ARG_" + arg, str(args[i + 1])) # type: ignore[index]
synapse_main | IndexError: tuple index out of range
```
