metrics: Expose decision log metrics

[anderseknert]

In order to better be able to analyze and troubleshoot decision log performance over time, it would be useful if OPA exposed metrics around that. Things like:

• Current size of decision log buffer (number of log messages and size).
• Decision packages sent per some given time frame.
• Number of packages failed to be sent.
• Number of decisions discarded (if configured to do that).

..and possibly other metrics of value.

[stale[bot]]

This issue has been automatically marked as inactive because it has not had any activity in the last 30 days.

[stale[bot]]

This issue has been automatically marked as inactive because it has not had any activity in the last 30 days. Although currently inactive, the issue could still be considered and actively worked on in the future. More details about the use-case this issue attempts to address, the value provided by completing it or possible solutions to resolve it would help to prioritize the issue.