Document getting the active/current span for each language SDK

[cartermp]

Tracking issue for the "Getting Current Span" row here: https://docs.google.com/spreadsheets/d/1EO0HkjraZSN_7LlFpwFPoazR9xbdCCgQi-IFc0YA5rU/edit#gid=0

Emoji guide:

✔️ - this is basically good
➖ - this could be improved
❌ - this needs to exist or be rewritten

Currently documented with an example:

• ✔️ Python (link)
• ✔️ JavaScript
• ✔️ Java
• ✔️ Ruby
• ✔️ Go

Has a section saying it exists, but needs an example of using at least one:

• ➖ C++ (doesn't have get, but has set, so it's pretty easy to figure it out from there)

Has no section at all:

• ❌ .NET
• ❌ Rust
• ❌ Erlang/Elixir
• ❌ Swift
• ❌ PHP

[chalin]

@cartermp - I added checkboxes to the opening comment for subtasks. Please mark items as done when they are completed, thanks.

[svrnm]

Can this be superseded by #3229?

[cartermp]

Yep