[loadbalancingexporter] Set the default otlp port 4317 in factory and unit test (open-telemetry#31425)

**Description:** 
As described in the otel-collector [issue
9505](open-telemetry/opentelemetry-collector#9505),
the otlpexporter does not function correctly if no port is defined. To
resolve this, the otlp config validation has been updated to fail fast
when the endpoint within an otlp config does not have a port specified.

The loadbalancingexporter config has the otlp exporter config as a
dependency, however, the loadbalancing exporter does not define a port
on the otlpexporter config in two places:
- default config from factory
- testdata contents

This is currently a blocker to the contrib tests for the
[PR](open-telemetry/opentelemetry-collector#9632)
to resolve issue 9505

Relates to:
open-telemetry/opentelemetry-collector#9523

open-telemetry#31371

open-telemetry#31381


**Link to tracking Issue:** 
otel-collector-contrib: [issue
31426](open-telemetry#31426)
Arises from otel-collector [issue
9505](open-telemetry/opentelemetry-collector#9505)

**Testing:** Used `replace` to test loadbalancingexporter changes pass
tests successfully when using the otlpexporter changes from
[PR](open-telemetry/opentelemetry-collector#9632)

Author: atmask

exporter/loadbalancingexporter/factory.go

@@ -32,7 +32,7 @@ func NewFactory() exporter.Factory {
 func createDefaultConfig() component.Config {
 	otlpFactory := otlpexporter.NewFactory()
 	otlpDefaultCfg := otlpFactory.CreateDefaultConfig().(*otlpexporter.Config)
-	otlpDefaultCfg.Endpoint = "placeholder"
+	otlpDefaultCfg.Endpoint = "placeholder:4317"
 
 	return &Config{
 		Protocol: Protocol{

exporter/loadbalancingexporter/testdata/test-build-exporter-config.yaml

@@ -7,7 +7,7 @@ exporters:
   loadbalancing:
     protocol:
       otlp:
-        endpoint: should-be-replaced
+        endpoint: should-be-replaced:4317
 
     resolver:
       static: