enhancement(loki sink): Adhere to InternalEvents spec (#14465)

* chore(loki sink): Remove duplicate code from healthcheck

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

* chore(loki sink): Misc cleanup and comments

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

* enhancement(loki sink): Adhere to errors and dropped events spec

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

* +missing punctuation in internal event

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>