improved Mandatory ID Filter to return info upon rejection

[sming]

• it now correctly displays a JSON error message when the request is anonymous.
• also refactored MandatoryClientId & Shutdown Filters using the Template Method design pattern as they were
  very similar.

Note for reviewers

Checkout out the Template Method design pattern before reviewing this PR 👍🏻

[codecov]

Codecov Report

Merging #751 (1ec4570) into master (7a830df) will increase coverage by 0.11%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master     #751      +/-   ##
============================================
+ Coverage     55.02%   55.13%   +0.11%     
- Complexity     3153     3162       +9     
============================================
  Files           746      748       +2     
  Lines         20389    20395       +6     
  Branches       1339     1339              
============================================
+ Hits          11219    11245      +26     
+ Misses         8682     8662      -20     
  Partials        488      488              

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com/spotify/heroic/http/HttpServer.java	16.52% <0.00%> (ø)	3.00 <0.00> (ø)
...ava/com/spotify/heroic/servlet/ShutdownFilter.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../java/com/spotify/heroic/servlet/SimpleFilter.java	88.88% <88.88%> (ø)	4.00 <4.00> (?)
...potify/heroic/servlet/MandatoryClientIdFilter.java	100.00% <100.00%> (+20.00%)	4.00 <2.00> (-1.00)	⬆️
...otify/heroic/ws/MandatoryClientIdErrorMessage.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
.../main/java/com/spotify/heroic/ws/ErrorMessage.java	57.14% <0.00%> (+57.14%)	4.00% <0.00%> (+4.00%)

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[lmuhlha]

worked like a charm locally:
{"message":"Anonymous requests are not permitted","reason":"Service Unavailable","status":503,"type":"authentication-error"}%
nice :)

[lmuhlha]

One question I had, why the switch from 400 to 503?

[sming]

One question I had, why the switch from 400 to 503?
that is a very good question, it should be 400. 503 is wrong. Weirdly though IIRC there's a test proving that it returns 400...

[sming]

Hey @lmuhlha , @samfadrigalan could one of you please approve - I addressed all the issues raised. Cheers.

[adsail]

@sming One quick thought -- It might be a good idea to update the public docs for /query/metrics to stipulate the header requirement. It's currently part of the sample cURL but not called out as a requirement.

https://github.com/spotify/heroic/blob/master/docs/content/_endpoints/post-query-metrics.md