Add the internal/observ package to otlptracegrpc (#7404)

- Part of #7007 
- Contains a TODO tracking features added in #7401 
- This package will be used to instrument `otlptracegrpc` in a follow-up
PR

### Benchmarks

```terminal
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc/internal/observ
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                                          │ otlptracegrpc-internal-observ.bmark.result │
                                          │                   sec/op                   │
InstrumentationExportSpans/NoError-8                                      143.8n ±  4%
InstrumentationExportSpans/PartialError-8                                 1.747µ ±  6%
InstrumentationExportSpans/FullError-8                                    1.737µ ± 11%
geomean                                                                   758.4n

                                          │ otlptracegrpc-internal-observ.bmark.result │
                                          │                    B/op                    │
InstrumentationExportSpans/NoError-8                                      0.000 ± 0%
InstrumentationExportSpans/PartialError-8                                 753.0 ± 0%
InstrumentationExportSpans/FullError-8                                    753.0 ± 0%
geomean                                                                              ¹
¹ summaries must be >0 to compute geomean

                                          │ otlptracegrpc-internal-observ.bmark.result │
                                          │                 allocs/op                  │
InstrumentationExportSpans/NoError-8                                      0.000 ± 0%
InstrumentationExportSpans/PartialError-8                                 4.000 ± 0%
InstrumentationExportSpans/FullError-8                                    4.000 ± 0%
geomean                                                                              ¹
¹ summaries must be >0 to compute geomean
```

---------

Co-authored-by: Robert Pająk <[email protected]>

Author: MrAlias

.lycheeignore

@@ -8,5 +8,6 @@ https?:\/\/github\.com\/open-telemetry\/semantic-conventions\/archive\/refs\/tag
 file:///home/runner/work/opentelemetry-go/opentelemetry-go/libraries
 file:///home/runner/work/opentelemetry-go/opentelemetry-go/manual
 http://4.3.2.1:78/user/123
+file:///home/runner/work/opentelemetry-go/opentelemetry-go/exporters/otlp/otlptrace/otlptracegrpc/internal/observ/dns:/:4317
 # URL works, but it has blocked link checkers.
 https://dl.acm.org/doi/10.1145/198429.198435

exporters/otlp/otlpmetric/otlpmetricgrpc/internal/partialsuccess.go

@@ -7,7 +7,9 @@ require (
 	github.com/stretchr/testify v1.11.1
 	go.opentelemetry.io/otel v1.38.0
 	go.opentelemetry.io/otel/exporters/otlp/otlptrace v1.38.0
+	go.opentelemetry.io/otel/metric v1.38.0
 	go.opentelemetry.io/otel/sdk v1.38.0
+	go.opentelemetry.io/otel/sdk/metric v1.38.0
 	go.opentelemetry.io/otel/trace v1.38.0
 	go.opentelemetry.io/proto/otlp v1.8.0
 	go.uber.org/goleak v1.3.0
@@ -24,7 +26,6 @@ require (
 	github.com/grpc-ecosystem/grpc-gateway/v2 v2.27.3 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect
-	go.opentelemetry.io/otel/metric v1.38.0 // indirect
 	golang.org/x/net v0.44.0 // indirect
 	golang.org/x/sys v0.36.0 // indirect
 	golang.org/x/text v0.29.0 // indirect