GH-475: Optional Feign clients eager initialization mechanism

[Periecle]

Pull Request majorly influenced by similar Ribbon feature, therefore API and configuration are close to each other for usability.
Also updated documentation explaining configuration and implementation details.
Fixes #475

[codecov]

Codecov Report

Merging #512 (cc04f56) into master (61aee7c) will increase coverage by 0.16%.
The diff coverage is 94.44%.

@@             Coverage Diff              @@
##             master     #512      +/-   ##
============================================
+ Coverage     77.85%   78.01%   +0.16%     
- Complexity      489      496       +7     
============================================
  Files            60       62       +2     
  Lines          1820     1838      +18     
  Branches        267      267              
============================================
+ Hits           1417     1434      +17     
- Misses          256      257       +1     
  Partials        147      147              

Impacted Files	Coverage Δ	Complexity Δ
...work/cloud/openfeign/FeignEagerLoadProperties.java	88.88% <88.88%> (ø)	4.00 <4.00> (?)
.../openfeign/FeignApplicationContextInitializer.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...mework/cloud/openfeign/FeignAutoConfiguration.java	82.81% <100.00%> (+0.55%)	4.00 <0.00> (ø)
.../springframework/cloud/openfeign/FeignContext.java	100.00% <100.00%> (ø)	4.00 <1.00> (+1.00)

[OlgaMaciaszek]

@Periecle thanks for submitting this. Have added some comments to address, however, have also run some tests on the sample provided in the issue (#475) and this does not really seem to solve the issue, so probably will require further investigation.

[Periecle]

@Periecle thanks for submitting this. Have added some comments to address, however, have also run some tests on the sample provided in the issue (#475) and this does not really seem to solve the issue, so probably will require further investigation.

@OlgaMaciaszek , author of original issue was struggling with initialising feign clients in async way from ForkJoinPool, and you proposed an alternative - implement something similar to ribbon.eager-load mechanism. I think my PR won't fix sample from original issue, but propose options to provide such functionality directly without need for such workarounds with supplying fake request to FJP or any other thread pool.
UPD: On second thought, it seems that cloud-openfeign need option for initialising Ribbon LB on ApplicationReadyEvent optionally on feign.ribbon.warmup, . Actually, I am already working on this feature.

[Periecle]

@OlgaMaciaszek, hi. Can you update me with current status of review?

[OlgaMaciaszek]

hi. Can you update me with current status of review?

@Periecle , I understood you were still working on this bit for the PR:

On second thought, it seems that cloud-openfeign need option for initialising Ribbon LB on ApplicationReadyEvent optionally on feign.ribbon.warmup, . Actually, I am already working on this feature.

Or will you be submitting it in another PR?