Move log.Processor.Enabled to independent FilterProcessor interfaced type (#5692)

Closes #5425 

Our current log `Processor` interface contains more functionality than
the [OTel
spec](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/sdk.md#logrecordprocessor-operations).
The additional functionality allows processors to report back to the API
if a Record should be constructed and emitted or not, which is quite
helpful[^1][^2][^3][^4][^5].

This removes the `Enabled` method from the `Processor` type. It adds
this functionality a new optional and experimental `FilterProcessor`
interface type. The logger and provider are updated to check for this
optional interface to be implemented with the configured processors and
uses them to back the `Logger.Enabled` method, preserving existing
functionality.

By making this change:

- The `Processor` interface is now compliant with the OTel spec and does
not contain any additional unspecified behavior.
- All `Processor` implementations are no longer required to implement an
`Enabled` method. The default, when they do not implement this method,
is to assume they are enabled.

### Benchmark

```terminal
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/log
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                │   old.txt    │              new7.txt               │
                │    sec/op    │   sec/op     vs base                │
LoggerEnabled-8   133.30n ± 3%   32.36n ± 3%  -75.72% (p=0.000 n=10)

                │  old.txt   │            new7.txt            │
                │    B/op    │    B/op     vs base            │
LoggerEnabled-8   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

                │  old.txt   │            new7.txt            │
                │ allocs/op  │ allocs/op   vs base            │
LoggerEnabled-8   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal
```

This is a significant performance improvement due to the `Record` no
longer being converted from the API version to the SDK version.

[^1]: https://pkg.go.dev/go.opentelemetry.io/contrib/processors/minsev
[^2]:
https://pkg.go.dev/go.opentelemetry.io/otel/sdk/log#BatchProcessor.Enabled
[^3]:
https://pkg.go.dev/go.opentelemetry.io/otel/sdk/log#SimpleProcessor.Enabled
[^4]:
af75717ac4/bridges/otelslog/handler.go (L206-L211)
[^5]:
d0309ddd8c/bridges/otelzap/core.go (L142-L146)

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>
Co-authored-by: Sam Xie <sam@samxie.me>

sdk/log/example_test.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"fmt"
 	"strings"
+	"sync"
 
 	logapi "go.opentelemetry.io/otel/log"
 	"go.opentelemetry.io/otel/log/global"
@@ -58,7 +59,7 @@ func ExampleProcessor_filtering() {
 	// Wrap the processor so that it ignores processing log records
 	// when a context deriving from WithIgnoreLogs is passed
 	// to the logging methods.
-	processor = &ContextFilterProcessor{processor}
+	processor = &ContextFilterProcessor{Processor: processor}
 
 	// The created processor can then be registered with
 	// the OpenTelemetry Logs SDK using the WithProcessor option.
@@ -81,6 +82,15 @@ func WithIgnoreLogs(ctx context.Context) context.Context {
 // [WithIgnoreLogs] is passed to its methods.
 type ContextFilterProcessor struct {
 	log.Processor
+
+	lazyFilter sync.Once
+	// Use the experimental FilterProcessor interface
+	// (go.opentelemetry.io/otel/sdk/log/internal/x).
+	filter filter
+}
+
+type filter interface {
+	Enabled(ctx context.Context, record log.Record) bool
 }
 
 func (p *ContextFilterProcessor) OnEmit(ctx context.Context, record *log.Record) error {
@@ -91,7 +101,12 @@ func (p *ContextFilterProcessor) OnEmit(ctx context.Context, record *log.Record)
 }
 
 func (p *ContextFilterProcessor) Enabled(ctx context.Context, record log.Record) bool {
-	return !ignoreLogs(ctx) && p.Processor.Enabled(ctx, record)
+	p.lazyFilter.Do(func() {
+		if f, ok := p.Processor.(filter); ok {
+			p.filter = f
+		}
+	})
+	return !ignoreLogs(ctx) && (p.filter == nil || p.filter.Enabled(ctx, record))
 }
 
 func ignoreLogs(ctx context.Context) bool {