[exporter/datadog] - Add debug flag to print payload data sent to Datadog API

[kmotupalli-tc]

Is your feature request related to a problem? Please describe.

When sending prometheus metrics to Datadog, some of the host metadata, labels and metrics are missing on Datadog.

Describe the solution you'd like

Logging payload data will help debugging this issue.

Describe alternatives you've considered

1. Log in stdout
2. Send it to DD logs instead of stdout
3. Log to file

[kmotupalli-tc]

0.37.1 version had similar debug logging which got removed in later releases. Want to add similar feature which can be enabled as flag if needed. #5856

[jpkrohling]

cc @KSerrania @mx-psi @gbbr @knusbaum

[mx-psi]

Sorry, I tagged this but didn't say anything :) We are tracking this internally, I removed the debugging because it was unintended and I want to avoid spammy logs by default, but it may be a good idea to add something like this behind a flag