Add guides for how to set API Keys

[blakeli0]

API Keys is supported in some services, especially maps. However, it's not clear to customers regarding how to set it up through client libraries. We should document the current way of setting API keys in README, similar to how we did it for RetrySettings. See related issue internally b/336630006 for more details.

[vedranstanic82]

this is pretty bad - you tell new customers to use the new places api, but the docs are missing and there are no clear instructions to use the API keys. 3 weeks ago? i hear its like this in the AI side of things (This day in AI). You make all this amazing technology, but it is set up like the core users of these sdks are Google Developers who are making the tech instead of the customers who live outside the Google world and examine your pricing pages. This is by far the easiest part of the project - documenting it for the end user.

[blakeli0]

@vedranstanic82 While I understand the frustration, please be ware of the languages used to describe the issue. I modified your comments to meet our community guidelines.

In addition, if you have a support contract with Google, please create an issue in the support console instead of filing on GitHub. This will ensure a more timely response.