Correct common question in the Starlark Processor README about shared state.

[thatsafunnyname]

Hello and thanks for telegraf.

After #8447 , this common question in the Starlark Processor README.md should be updated to reflect the newly available shared state:

https://github.com/influxdata/telegraf/blame/8ddbab47a46e256281392ad0aac876715189c117/plugins/processors/starlark/README.md#L166

How can I save values across multiple calls to the script?

Telegraf freezes the global scope, which prevents it from being modified.
Attempting to modify the global scope will fail with an error.

Suggest replacing this with:

A shared global dictionary named "state" exists, this can be used by the apply function.
See an example of this in plugins/processors/starlark/testdata/compare_metrics.star

Thanks

[sjwang90]

@thatsafunnyname would you want to submit a PR for changes to the README?

[thatsafunnyname]

I can do, I just need to wait for approval from my employer to sign the Individual CLA.