Add OTLP exporter #toBuilder() methods to ease autoconfigure customization

[jack-berg]

AutoConfigurationCustomizer supports the ability to customize exporters using methods like addSpanExporterCustomizer.

Maybe you want to use environment variables to configure the OTLP exporters, but add further customization for parameters which aren't configurable (like retry, or custom default metric aggregation). So you'd expect to do something like:

    customizer.addSpanExporterCustomizer(
        (exporter, config) -> {
          if (exporter instanceof OtlpGrpcSpanExporter) {
            return OtlpGrpcSpanExporter.builder()
                .setEndpoint("http://foo.com")
                .setRetryPolicy(RetryPolicy.builder().setMaxAttempts(2).build())
                .build());
          }
          return exporter;
        });

The problem is that you can't access / extend the OTLP exporter parameters which were configured via environment variables, so you're essentially forced into doing all programatic configuration.

I propose we add OtlpGrpcSpanExporter toBuilder() methods to to each of the OTLP exporters. This would allow users to extend the environment variable based config like:

    customizer.addSpanExporterCustomizer(
        (exporter, config) -> {
          if (exporter instanceof OtlpGrpcSpanExporter) {
            exporter.shutdown().join(10, TimeUnit.SECONDS);
            return exporter.toBuilder()
                .setRetryPolicy(RetryPolicy.builder().setMaxAttempts(2).build())
                .build();
          }
          return exporter;
        });

Notice that the user will have to be careful to close the previous exporter before creating a new one, else they'll have unclosed resources like thread pools floating around.