Use confighttp.NewDefaultClientConfig instead of manually creating struct

[mackjmr]

Component(s)

No response

Describe the issue you're reporting

Related to second point in: https://github.com/open-telemetry/opentelemetry-collector/issues/9478#issuecomment-2241891068.

Move away from manually creating confighttp.ClientConfig in favor of using confighttp.NewDefaultClientConfig.

This is necessary for the following components:

• [x] alertmanagerexporter
• [x] datadogexporter
• [ ] influxdbexporter
• [ ] logzioexporter
• [ ] lokiexporter
• [ ] memzoexporter
• [ ] prometheusremotewriteexporter
• [ ] signalfxexporter
• [ ] splunkhecexporter
• [ ] sumologicexporter
• [ ] internal/aws/ecsutil
• [ ] pkg/datadog/config
• [ ] apachereceiver
• [ ] apachesparkreceiver
• [ ] awscontainerinsightreceiver
• [ ] awsecscontainermetricsreceiver
• [ ] bigipreceiver
• [ ] cloudfoundryreceiver
• [ ] couchdbreceiver
• [ ] elasticsearchreceiver
• [ ] expvarreceiver
• [ ] flinkmetricsreceiver
• [ ] githubreceiver
• [ ] nginxreceiver
• [ ] purefarereceiver
• [ ] purefbreceiver
• [ ] rabbitmqreceiver
• [ ] riakreceiver
• [ ] simpleprometheusreceiver

This will be done following conditions:

• Fields that are currently different than the default will be updated after call to NewDefaultClientConfig
• Fields that are same as default will not be set manually anymore
• Fields that are set in the default but currently unset in above components will be either manually unset (for backwards compatibility) or will be updated to match default.

For the third point, I am unsure of which approach to follow. An example is the otlphttpexporter (#11273), where previously MaxIdleConns, MaxIdleConnsPerHost, MaxConnsPerHost and IdleConnTimeout were unset (equivalent of nil), which is different than the default which has values.

Core collector issue: https://github.com/open-telemetry/opentelemetry-collector/issues/11274

[crobert-1]

Removing needs triage, sounds like a good idea to me.