Influxdb 2.0.9 Linear Regression Error

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Setup multiple tables using any data source to perform a linear regression on.
2. import contrib/anaisdg/statsmodels, and attempt to run linear regression on more than one table at once
3. View the slope values, and other relevant information for each table.

__Expected behavior:__
Linear regression function is performed independently on each table, producing the slope that is correct for the table that is being evaluated, individually from the other tables.

__Actual behavior:__
When running the linear regression function, it evaluates all of the tables at once, taking in all of the points to determine the slope, and other relevant values.

__Environment info:__

* System info: Run `uname -srm` and copy the output here
* Linux 5.13.0-30-generic x86_64
* InfluxDB version: Run `influxd version` and copy the output here
* InfluxDB 2.0.9 Alpine
* Other relevant environment details: Container runtime, disk info, etc

__Config:__

__Logs:__

__Performance:__

```sh
# Commands should be run when the bug is actively happening.
# Note: This command will run for ~30 seconds.
curl -o profiles.tar.gz "http://localhost:8086/debug/pprof/all?cpu=30s"
iostat -xd 1 30 > iostat.txt
# Attach the `profiles.tar.gz` and `iostat.txt` output files.
```


Start by reproducing the behavior described with multiple tables and the linear regression function from contrib/anaisdg/statsmodels on InfluxDB 2.0.9. Compare the slope and related values for each table with the combined result. Done means regression is performed independently for every table.

Written by the indexing model from the issue text.