Allow configuring header extraction via runtime parameters

[adriangb]

Currently, web framework instrumentations only allow configuring header extraction via environment variables. I want to add the option to configure them via constructor parameters. I think the code should be quite straightforward but I want to check if this would be an acceptable change before making a PR. Some background on why it wasn't implemented in the first place might also be helpful if it's something other than 'no one asked for it yet'.

[adriangb]

@lzchen @srikanthccv any thoughts? I don't see any reason why it couldn't be added as a runtime parameter as well.

[srikanthccv]

I think we followed what Java did at the time and used the same environment variables for consistency. We already support configuring some capabilities such as excluding URLs with construction params. This change is fine. Please send a pull request.

[adriangb]

Thank you @ocelotl for merging the PR! Do you have any preferenece on how we pass these parameters to the Starlette and FastAPI middleware?

[srikanthccv]

Usually, we pass down the parameters as we do for the *_hook today. Do you have some other ideas?