Add function memoization

[JeanMertz]

There have been several situations in which a VRL function has relatively slow runtime performance, but the operation it performs results in a value that is relatively static.

see: https://github.com/timberio/vector/pull/6141

I propose we add a “function cache” to the VRL runtime.

The idea is this:

• We add a new state::FunctionCache object.
• Functions have access to their own piece of this cache, namespaces by the function identifier (to start)
• They can store a Value type in the cache to fetch when needed.
• We either implement a TTL in the cache, or a “query counter”. Alternatively we leave cache invalidation up to the function.

There are still some things to work out here, but it seems a valuable property to have in the runtime.

[bruceg]

Memoization like this makes sense for really slow functions, but we should be careful of two things:

1. Hashing parameters to access the cached value and cloning to store a new value also has a cost, so this should probably be limited to certain functions that are known to be slow.
2. Make sure there is at least consideration for excluding functions that are not const.

[fpytloun]

I am also very interested in this one. Any time soon? 🙂

[pront]

I am also very interested in this one. Any time soon? 🙂

We do not have capacity in the near future. This sounds like a medium size project and will some thought on how to integrate this in the VRL runtime.

Contributions are always welcome.