Create global tag setting for using local timestamp

[sjwang90]

Feature Request

Continuation of #8411

Proposal:

Create a global tag that can be set to true that will allow you to using the collector's local time instead of using the timestamp of the notification/device.

Current behavior:

Data received over gnmi are always timestamped using the Timestamp field from the device.

Desired behavior:

Setting a tag to use local collectors timestamp instead of device's timestamp.

Use case:

There are a lot of use cases where you don't want to use the device's timestamp and want to use local collector's timestamp. This is the case for many plugins and setups, not just gnmi, so we prefer to add a global tag (ex: use_local_timestamp = true) that can be used across Telegraf.

Closes #8411

[sjwang90]

The solution for this will be to utilize the Starlark processor. There is this example script that can be added to use the current/local timestamp over the field device's timestamp.