[exporter/datadog] Add metrics::summaries::mode setting; deprecate metrics::report_quantiles

[mx-psi]

Description:

• Add new metrics::summaries::mode setting.
• Deprecate metrics::report_quantiles in favor of metrics::summaries::mode.

Link to tracking Issue: #8845

Testing: Add unit tests for invalid mode; test manually. The existing unit tests for cumulative_monotonic_mode are enough to check the deprecation system

Documentation: Updated the docs to reflect the new option

[jpkrohling]

Interesting, the conflict I have when checking out this PR is different:

Unmerged paths:
  (use "git restore --staged <file>..." to unstage)
  (use "git add <file>..." to mark resolution)
	both modified:   exporter/datadogexporter/factory_test.go

Right now, GitHub tells me the conflict is at the changelog. @mx-psi, could you please take a look?

[mx-psi]

Interesting, the conflict I have when checking out this PR is different:

Unmerged paths:
  (use "git restore --staged <file>..." to unstage)
  (use "git add <file>..." to mark resolution)
	both modified:   exporter/datadogexporter/factory_test.go

Right now, GitHub tells me the conflict is at the changelog. @mx-psi, could you please take a look?

I fixed that on 0817d87, I think your branch was outdated. Anyway, all conflicts fixed now!

[KSerrania]

LGTM

[mx-psi]

@jpkrohling if everything looks good, could this be merged?