Unify metric SDK caching

[MrAlias]

Currently, the only place caching is used is the meterRegistry.

There are currently two reported bugs (#3229 and #3240) that would be resolved by adding caching to instrument aggregation determination. If done correctly the same request for compatible instruments would return the same Aggregator instance and conflicting, but resolvable, instruments would also get the same instance.

Finally, there is an optimization opportunity at the instrument provider level to cache returned instruments. Adding a cache at this level would prevent duplicate resolutions of aggregations.

Propoasal

A proof-of-concept with all three caching situations can be found here: https://github.com/MrAlias/opentelemetry-go/pull/662/files

The proposed approach is to add a unified cache type that handles synchronized lookup in a map:

type cache[K comparable, V any] struct {
	sync.Mutex
	data map[K]V
}

func (c *cache[K, V]) Lookup(key K, f func() V) V {
	c.Lock()
	defer c.Unlock()

	if c.data == nil {
		val := f()
		c.data = map[K]V{key: val}
		return val
	}
	if v, ok := c.data[key]; ok {
		return v
	}
	val := f()
	c.data[key] = val
	return val
}

This type can be wrapped to provide convenient lookup of aggregator caching: https://github.com/MrAlias/opentelemetry-go/blob/0d925dcfc651811318155a16bcc59c615ff119cf/sdk/metric/cache.go#L71-L147

Or it can be used to directly cache meter or instrument values.

The plan is to break this work into 3 PRs:

• Add the cache type and cache instrument aggregators: Handle duplicate Aggregators and log instrument conflicts #3251
• Replace the meterRegistry with the cache type: Replace meterRegistry with cache #3255
• Cache instrument provider results
  • Restructure instrument creation code paths #3256
  • Add caching to embedded instrument provider type added ^

[MadVikingGod]

@MrAlias Can you elaborate on the final bullet point?

[MrAlias]

@MrAlias Can you elaborate on the final bullet point?

Create a cache and use it for this method:

opentelemetry-go/sdk/metric/instrument_provider.go

Line 37 in 4146bd1

func (p *instProvider[N]) lookup(kind InstrumentKind, name string, opts []instrument.Option) (*instrumentImpl[N], error) {

[MrAlias]

We should split off this level of caching for its own issue and assign to the post-GA phase.

[MrAlias]

Remaining task split into #3527