Make headers on operations client injectable

[fables-tales]

For Google Ads we send additional headers for LRO reloads, specifically:

developer-token
login-customer-id

without these headers being injectable it's not possible for us to complete LRO calls successfully. This patch exposes headers in the constructor of the operations client so that we can pass these when we construct our operations client objects.

[codecov[bot]]

Codecov Report

Merging #200 into 1.x will decrease coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##              1.x     #200      +/-   ##
==========================================
- Coverage   99.29%   99.24%   -0.05%     
==========================================
  Files          19       19              
  Lines        2123     2123              
==========================================
- Hits         2108     2107       -1     
- Misses         15       16       +1

Impacted Files	Coverage Δ
spec/google/gax/bundling_spec.rb	99.71% <0%> (-0.29%)	:arrow_down:

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Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update c4460b5...963e391. Read the comment docs.

[codecov[bot]]

Codecov Report

Merging #200 into 1.x will decrease coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##              1.x     #200      +/-   ##
==========================================
- Coverage   99.29%   99.24%   -0.05%     
==========================================
  Files          19       19              
  Lines        2123     2123              
==========================================
- Hits         2108     2107       -1     
- Misses         15       16       +1

Impacted Files	Coverage Δ
spec/google/gax/bundling_spec.rb	99.71% <0%> (-0.29%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update c4460b5...963e391. Read the comment docs.

[dazuma]

@samphippen Just to be clear, this is for the current gapic generated clients, not the new generator, right? Do you have a similar requirement for the new generator?

[fables-tales]

@dazuma yes, we would need this in the new generator, but we need this for this generator right now for our LROs to work.

[fables-tales]

I dug in to this further, and I think there's a bug with per-rpc headers: https://github.com/googleapis/gax-ruby/blob/1.x/lib/google/gax/settings.rb#L128

This line of code

metadata = (metadata.dup if metadata) || {}

is attempting to use the metadata attr_accessor on the Settings instance. However, in ruby, creating a local variable implicitly binds it to nil in the body of the block:

p foo
foo = 3

will print nil for example. This local binding also beats methods of the same name. As such metadata on that line will always reference the local variable metadata which will be bound to nil, and as such this entire expression will always assign the empty hash from the || {} part of the line.

[blowmage]

Yeah, I can confirm that line is not working as expected. It looks like changing that line to metadata = @metadata.dup || {} would fix it.

[blowmage]

Closing in favor of #205.