`Configuration.setServerVariables` doesn't work with non-defaults server indices

[segevfiner]

Describe the bug
Configuration.setServerVariables doesn't work with non-defaults server indices. Either specified via serverIndex or via operationIndices

??? 😕 (Users have no permissions to do that...)

Label the issue properly.

• Add severity/ label.
• Add documentation label if this issue is related to documentation changes.

To Reproduce
Steps to reproduce the behavior:

1. Call Configuration.setServerVariables when the serverIndex or operationServerIndices are specified.

Expected behavior
It will set the variable to the operation servers as well. But it doesn't.

Environment and Versions (please complete the following information):
A clear and precise description of your setup:

• v1.25.0
• Via pino-datadog-transport

Additional context
I'm really only opening this to open a PR...