Make Prometheus endpoint take Accept header into account

[mrkloan]

Summary

closes #4673 by adding a wildcard @Produces on the /otp/actuators/prometheus endpoint method.

[codecov-commenter]

Codecov Report

Base: 60.86% // Head: 60.86% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (9a41d19) compared to base (1f6f0de).
Patch has no changes to coverable lines.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #4674      +/-   ##
=============================================
- Coverage      60.86%   60.86%   -0.01%     
  Complexity     12333    12333              
=============================================
  Files           1594     1594              
  Lines          63814    63814              
  Branches        6998     6998              
=============================================
- Hits           38843    38841       -2     
- Misses         22794    22796       +2     
  Partials        2177     2177              

Impacted Files	Coverage Δ
.../org/opentripplanner/ext/actuator/ActuatorAPI.java	0.00% <ø> (ø)
...aph_builder/module/OsmBoardingLocationsModule.java	78.82% <0.00%> (-2.36%)	⬇️
...ripplanner/model/transfer/ConstrainedTransfer.java	83.33% <0.00%> (ø)

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[mrkloan]

Opened to suggestions on how to best test this.

[hannesj]

We should probably always return text/plain; version=0.0.4, as described in the documentation

[mrkloan]

We should probably always return text/plain; version=0.0.4, as described in the documentation

You are right; in my latest commit I added support for both Prometheus and OpenMetrics format based on the Accept request header, and the endpoint now produces the exact Content-Type used in the response.

[leonardehrenfried]

BTW, if BlahBlahCar is using OTP in production, we would love to hear about it. Lots of core developers chat on https://gitter.im/opentripplanner/OpenTripPlanner and we have video meetings twice a week.