add new hooks for instrumenting dataloaders

[Emily]

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• [x] Please read our contributor guide
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• [x] Make sure the PR doesn't introduce backward compatibility issues
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Pull Request type

• [ ] Bugfix
• [x] Feature
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Other (please describe):

Changes in this PR

add new hooks for instrumenting dataloaders

• DgsDataLoaderScanningInterceptor: allows for wrapping a dataloader as it is initially being registered, including changing the type of the data loader
• DgsSimpleDataloaderInstrumentation: an even easier way to add instrumentation like metrics, tracing, etc...

also note: it is likely that in the future this will be able to simplify the current micrometer metrics instrumentation that uses ByteBuddy.

[paulbakker]

I'm curious if you've tried to replace the bytebuddy stuff with this. At the very least it would be a good way to test the new API, see if it's possible or what you run into.

I generally like the approach with the two different interfaces that happen at different times in the lifecycle.

[Emily]

@kilink @srinivasankavitha @paulbakker PTAL

@paulbakker I haven't yet, as the tests for the data loader metrics are currently disabled. Re-enabling them looks to be moderately involved so I was going to do that and the new style of instrumentation together as a follow up.