telemetry configuration causes crash at startup

[codeboten]

As noted by @skandragon in #12332, there is a bug in v0.119.0 and v0.120.0 that causes the following panic at start when more than one headers is configured:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xda8c5a]
goroutine 1 [running]:
go.opentelemetry.io/contrib/config/v0%2e3%2e0.toStringMap(...)
	go.opentelemetry.io/contrib/config@v0.14.0/v0.3.0/config.go:157
go.opentelemetry.io/contrib/config/v0%2e3%2e0.otlpGRPCMetricExporter({0x75e6e88, 0xb3cce60}, 0xc0005aa9a0)
	go.opentelemetry.io/contrib/config@v0.14.0/v0.3.0/metric.go:233 +0x61a
go.opentelemetry.io/contrib/config/v0%2e3%2e0.periodicExporter({0x75e6e88?, 0xb3cce60?}, {0x0?, 0xc0005aa9a0?, {0x0?, 0x0?}}, {0xc001421390, 0x1, 0x1})
	go.opentelemetry.io/contrib/config@v0.14.0/v0.3.0/metric.go:127 +0x1e6
go.opentelemetry.io/contrib/config/v0%2e3%2e0.metricReader({0x75e6e88, 0xb3cce60}, {0xc000e515c0, {0x0, 0x0, 0x0}, 0x0})
	go.opentelemetry.io/contrib/config@v0.14.0/v0.3.0/metric.go:88 +0x23a
go.opentelemetry.io/contrib/config/v0%2e3%2e0.meterProvider({{0x75e6e88, 0xb3cce60}, {0x0, 0x0, 0x0, 0x0, 0xc0014252e0, 0xc0015098c0, 0x0, 0xc001509920, ...}}, ...)
	go.opentelemetry.io/contrib/config@v0.14.0/v0.3.0/metric.go:50 +0x1bf
go.opentelemetry.io/contrib/config/v0%2e3%2e0.NewSDK({0xc00128f028?, 0x0?, 0x0?})
	go.opentelemetry.io/contrib/config@v0.14.0/v0.3.0/config.go:90 +0x1fe
go.opentelemetry.io/collector/service.New({_, _}, {{{0x6c1c2ff, 0x19}, {0x0, 0x0}, {0x0, 0x0}}, 0xc000ed7eb0, 0xc0009df9e0, ...}, ...)
	go.opentelemetry.io/collector/service@v0.119.0/service.go:140 +0x95c
go.opentelemetry.io/collector/otelcol.(*Collector).setupConfigurationComponents(0xc001086c60, {0x75e6e88, 0xb3cce60})
	go.opentelemetry.io/collector/otelcol@v0.119.0/collector.go:186 +0x588
go.opentelemetry.io/collector/otelcol.(*Collector).Run(0xc001086c60, {0x75e6e88, 0xb3cce60})
	go.opentelemetry.io/collector/otelcol@v0.119.0/collector.go:285 +0x55
go.opentelemetry.io/collector/otelcol.NewCommand.func1(0xc00080db08, {0x685a762?, 0x4?, 0x685a6da?})
	go.opentelemetry.io/collector/otelcol@v0.119.0/command.go:36 +0x94
github.com/spf13/cobra.(*Command).execute(0xc00080db08, {0xc000072050, 0x1, 0x1})
	github.com/spf13/cobra@v1.8.1/command.go:985 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0xc00080db08)
	github.com/spf13/cobra@v1.8.1/command.go:1117 +0x3ff
github.com/spf13/cobra.(*Command).Execute(0x6dfcdc8?)
	github.com/spf13/cobra@v1.8.1/command.go:1041 +0x13
main.runInteractive({0x6dfcdc8, {{0x6c1c2ff, 0x19}, {0x0, 0x0}, {0x0, 0x0}}, 0x0, {{{0x0, 0x0, ...}, ...}}, ...})
	go.opentelemetry.io/collector/cmd/builder/main.go:55 +0x45
main.run(...)
	go.opentelemetry.io/collector/cmd/builder/main_others.go:10
main.main()
	go.opentelemetry.io/collector/cmd/builder/main.go:48 +0x365

The case here was using "headers" where it was a map[string]string rather than a list of objects with {name, value}

Originally posted by @skandragon in #12332

[codeboten]

It took me some time to reproduce the issue, the following config causes can reproduce the problem:

service:
  telemetry:
        logs:
          processors:
          - batch:
              exporter:
                otlp:
                  endpoint: collector:4318
                  headers:
                    key1: value1
                    key2: value2
                    key3: value3
                  protocol: http/protobuf

[codeboten]

A workaround if to use the following config:

service:
  telemetry:
        logs:
          processors:
          - batch:
              exporter:
                otlp:
                  endpoint: collector:4318
                  headers:
                    - name: key1
                      value: value1
                    - name: key2
                      value: value2
                    - name: key3
                      value: value3
                  protocol: http/protobuf