[exporter/otlp] gRPC balancer default behavior

[tank-500m]

Component(s)

exporter/otlp

Describe the issue you're reporting

It looks like the OTLP exporter’s default load balancing is intended to be round_robin (see the related discussion: #10298).

However, due to the code around this line, the default can end up behaving like pick_first: factory.go
In addition, unless the change to use NewClient is completed (see #13632), the resolver scheme remains passthrough, which means round_robin will not actually take effect.

Proposed action:

• either call resolver.SetDefaultScheme("dns") so the DNS resolver is used, or complete the NewClient migration so that the proper resolver scheme is applied and round_robin works as intended.
  ref. go-grpc resolver
• Remove this line

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