fix: [PSERV-2110] skip_n_calls take amount accounts for current call

[LeweyM]

Description

This PR fixes an issue where skip_n_calls would inaccurately limit received requests.

Local Test Results

Tests generated locally using testing tools added here: https://github.com/auth0/platform-core-services-tools/pull/24/files

The graphs below show the number of conformant requests under the following conditions:

- 10 nodes making requests at 330 RPS (3300 total)
- global rate limit configured to 1000 RPS

Before Fix

Notice that it is limiting to 1.4k RPS instead of 1k RPS

After Fix

The graph below shows the number of conformant requests under the following conditions:

- 10 nodes making requests at 330 RPS (3300 total)
- global rate limit configured to 1000 RPS

Notice that it is now correctly limiting to 1k RPS

Other Local Test Case Results

Nodes	Limit Per Second	skip_n_calls	Rough Estimate of Allowed RPS
10	1000	0	997.6567443255674
10	1000	10	1014.9835699962139
10	1000	20	1031.8201798598202
10	2000	0	2008.1796153717962
10	2000	10	1975.5858597281813
10	2000	20	2009.101272813409
20	1000	0	1004.0967691042822
20	1000	10	1026.260158211675
20	1000	20	967.3338773895454
20	2000	0	2002.9174408932795
20	2000	20	1986.750242652974
30	1000	10	951.6324449810605

Root Cause:

When taking tokens after skipping calls to redis, we were taking tokens to account for the skipped calls, but not for the current call.

This off-by-one error explains why the results were more prominent when `n` is small.

Examples

When skip_n_calls is 1 we would do the following:

	Before	After
	(bucket = 3)	(bucket = 3)
call 1	take 1 token from redis - (bucket = 2)	take 1 token from redis (bucket = 2)
call 2	skip (bucket = 2)	skip (bucket = 2)
call 3	take n tokens from redis. n = 1. (bucket = 1)	take n + 1 tokens from redis; one for the previously skipped call, and one for this call. (bucket = 0)

When skip_n_calls is 3 we would do the following:

	Before	After
	(bucket = 5)	(bucket = 5)
call 1	take 1 token from redis - (bucket = 4)	take 1 token from redis (bucket = 4)
call 2	skip (bucket = 4)	skip (bucket = 4)
call 3	skip (bucket = 4)	skip (bucket = 4)
call 4	skip (bucket = 4)	skip (bucket = 4)
call 5	take n tokens from redis. n = 3. (bucket = 1)	take n + 1 tokens from redis; one for the previously skipped call, and one for this call. (bucket = 0)

References

Slack Discussion
Jira

Testing

• Integration testing
• Dev Environment Load Testing

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch