Multiple Flow-Control Key Per Publish

[sancar]

Users want to be able to combine flow control keys.
For example, suppose there are two endpoints that share the same database pool, and together they should not exceed a total parallelism of 50:

A - parallelism: 50, rate: 10  
B - parallelism: 50, rate: 20

If we define two separate flow control configurations (each with parallelism: 50) for A and B, the combined usage could exceed the required limit.
Instead, we can define separate flow control keys like this:

[
  { key: "dbTransaction", parallelism: 50 },
  { key: "someExternalApi", rate: 10 },
  { key: "anotherExternalApi", rate: 20 }
]

Then, we can assign combined flow control configurations to each endpoint:

A - ["dbTransaction", "someExternalApi"]
B - ["dbTransaction", "anotherExternalApi"]
C - ["dbTransaction"]

This ensures that all flow control rules sharing the "dbTransaction" key collectively stay within the parallelism: 50 limit.

[willdspd]

+1000! This is essential for adhering to many external API limits in a single endpoint.