Use the cause of the context error in OTLP retry

[XSAM]

Part of #6588

For a demo code like this

package main

import (
	"context"
	"fmt"
	"log"
	"time"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
)

func main() {
	ctx := context.Background()

	exp, err := newExporter(ctx)
	if err != nil {
		log.Fatalf("failed to initialize trace exporter: %v", err)
	}

	tp, err := newTracerProvider(exp)
	if err != nil {
		log.Fatalf("failed to initialize trace provider: %v", err)
	}

	defer func() { _ = tp.Shutdown(ctx) }()

	otel.SetTracerProvider(tp)
	generateSpan()

	select {}
}

func generateSpan() {
	log.Println("Generating a dummy span")
	_, span := otel.Tracer("").Start(context.Background(), "dummy")
	defer span.End()
}

func newTracerProvider(exp sdktrace.SpanExporter) (*sdktrace.TracerProvider, error) {
	return sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exp),
	), nil
}

func newExporter(ctx context.Context) (*otlptrace.Exporter, error) {
	traceExporter, err := otlptrace.New(
		ctx,
		otlptracegrpc.NewClient(
			otlptracegrpc.WithEndpoint("127.0.0.1:4317"),
			otlptracegrpc.WithInsecure(),
			otlptracegrpc.WithRetry(otlptracegrpc.RetryConfig{
				Enabled:         true,
				InitialInterval: 1 * time.Second,
				MaxInterval:     30 * time.Second,
				MaxElapsedTime:  time.Minute,
			}),
		),
	)
	if err != nil {
		return nil, fmt.Errorf("failed to create trace exporter: %w", err)
	}

	return traceExporter, nil
}

the error result from

traces export: context deadline exceeded: rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing: dial tcp 127.0.0.1:4317: connect: connection refused"

become

traces export: exporter export timeout: rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing: dial tcp 127.0.0.1:4317: connect: connection refused"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.3%. Comparing base (55c7a70) to head (78e36c8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6898   +/-   ##
=====================================
  Coverage   82.2%   82.3%           
=====================================
  Files        263     263           
  Lines      24418   24420    +2     
=====================================
+ Hits       20096   20101    +5     
+ Misses      3940    3937    -3     
  Partials     382     382           

Files with missing lines	Coverage Δ
exporters/otlp/otlplog/otlploggrpc/client.go	91.9% <100.0%> (ø)
...s/otlp/otlplog/otlploggrpc/internal/retry/retry.go	98.2% <100.0%> (ø)
...s/otlp/otlplog/otlploghttp/internal/retry/retry.go	98.2% <100.0%> (ø)
exporters/otlp/otlpmetric/otlpmetricgrpc/client.go	91.8% <100.0%> (ø)
.../otlpmetric/otlpmetricgrpc/internal/retry/retry.go	98.2% <100.0%> (ø)
.../otlpmetric/otlpmetrichttp/internal/retry/retry.go	98.2% <100.0%> (ø)
exporters/otlp/otlptrace/otlptracegrpc/client.go	91.1% <100.0%> (ø)
...lp/otlptrace/otlptracegrpc/internal/retry/retry.go	98.2% <100.0%> (ø)
...lp/otlptrace/otlptracehttp/internal/retry/retry.go	98.2% <100.0%> (ø)
sdk/log/exporter.go	97.1% <100.0%> (-0.7%)	⬇️
... and 2 more

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[pellared]

It would be nice to check if there are existing timeout tests and if yes then we can additionally assert that the returned error contains the cause.