Expose LRO callables for HttpJson

[andreamlin]

Just the HttpJson version of GrpcStubCallableFactory and GrpcCallableFactory.

These new methods will be implemented by future versions of HttpJsonGlobalOperationStubCallableFactory, etc., in the same way that com.google.longrunning.stub.GrpcOperationsCallableFactory implements GrpcStubCallableFactory.

[andreamlin]

PTAL

[codecov]

Codecov Report

Merging #684 into master will decrease coverage by 0.13%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #684 +/- ##
===========================================
- Coverage 75.63% 75.5% -0.14% 
Complexity 1029 1029 ===========================================
Files 195 195 Lines 4613 4621 +8 Branches 356 356 ===========================================
Hits 3489 3489 - Misses 966 974 +8 
Partials 158 158

Impacted Files	Coverage Δ	Complexity Δ
...ogle/api/gax/httpjson/HttpJsonCallableFactory.java	24.24% <0%> (-7.76%)	3 <0> (ø)

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[vam-google]

LGTM

[vam-google]

Minor: should Unary start with capital letter here?

[codecov-io]

Codecov Report

Merging #684 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #684 +/- ##
============================================
- Coverage 75.63% 75.51% -0.12% - Complexity 1029 1030 +1 
============================================
Files 195 195 Lines 4613 4616 +3 Branches 356 357 +1 ============================================
- Hits 3489 3486 -3 - Misses 966 972 +6 
Partials 158 158

Impacted Files	Coverage Δ	Complexity Δ
...ogle/api/gax/httpjson/HttpJsonCallableFactory.java	24.24% <0%> (-7.76%)	3 <0> (ø)
...a/com/google/api/gax/tracing/OpencensusTracer.java	87.5% <0%> (+0.29%)	32% <0%> (+1%)	⬆️
...api/gax/tracing/TracedServerStreamingCallable.java	100% <0%> (+8.69%)	2% <0%> (ø)	⬇️

---

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