Prevent an exception with large num_retries.

If the calculated wait time is infinite, don't try to do
further calculations on it to prevent it from becoming NaN.
This could happen if num_retries becomes too large (>1024).
Refactor in_monitor_agent to remove unnecessary instance eval
reorder conditional to handle NaN or Infinite
Merge pull request #502 from mr-salty/fix-max-retry-exception

Prevent an exception with large num_retries.
Accept '@type' and '@id' in v1 configuration. fixes #500

Plugin type and id are system reserved parameters like @label.
So using '@' prefix is better for separating normal plugin parameters.
in_monitor_agent supports '@type' and '@id' query
Merge pull request #503 from fluent/at-prefix-for-type-and-id-parameters

Accept '@type' and '@id' in v1 configuration. fixes #500