command: Create separate tracing span when acquiring spawn tokens

When trying to dig into performance-related issues, one frequently sees trace spans where commands are seemingly hanging for a long time while doing nothing. More often than not, the root cause of this is that too many requests come in and as a result have emptied available spawn tokens. While this becomes visible as soon as one takes a look at logs, it is highly misleading when only taking a look at trace spans.

Create a separate span when retrieving spawn tokens to better highlight what's happening and avoid confusion.