Alter Preprocess bag handling to prevent attribute overriding

[douglas-reid]

Prevent APAs from overriding attributes sent in the original request.

This adds a new method to MutableBag named PreserveMerge which takes in bags that possibly contain conflicts and does a merge in which original values are maintained and conflicts are silently dropped on the floor.

This is in response to errors reported on the dev mailing list that stemmed from having multiple values for the attribute destination.service in the attribute bag that was used to create instances.

Release note:

Prefer received attribute values to preprocess-derived attribute values.

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This change is 

[istio-testing]

@douglas-reid: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

One of the following labels is required "release-note", "release-note-action-required", or "release-note-none". Please see: https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md#write-release-notes-if-needed. Instructions for interacting with me using PR comments are available [here](https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[douglas-reid]

/hold

[geeknoid]

/lgtm

[istio-merge-robot]

/lgtm cancel //PR changed after LGTM, removing LGTM. @douglas-reid @geeknoid

[douglas-reid]

PTAL. Updated code to address linter issues. This required a small set of changes around error-handling, but I think that is OK given initial feedback.

[geeknoid]

Groovy.

/lgtm

[istio-merge-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: geeknoid

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these OWNERS Files: - ~~[OWNERS](https://github.com/istio/mixer/blob/master/OWNERS)~~ [geeknoid] You can indicate your approval by writing `/approve` in a comment You can cancel your approval by writing `/approve cancel` in a comment

[istio-merge-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[istio-merge-robot]

Automatic merge from submit-queue.

[codecov[bot]]

Codecov Report

Merging #1460 into master will decrease coverage by 0.08%. The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master    #1460      +/-   ##
==========================================
- Coverage   84.66%   84.58%   -0.09%     
==========================================
  Files         122      122              
  Lines       11922    11980      +58     
==========================================
+ Hits        10094    10133      +39     
- Misses       1632     1646      +14     
- Partials      196      201       +5

Impacted Files	Coverage Δ
pkg/api/grpcServer.go	76.17% <62.5%> (-2%)	:arrow_down:
pkg/attribute/mutableBag.go	89.71% <64.7%> (-1.71%)	:arrow_down:
adapter/stackdriver/metric/bufferedClient.go	90% <0%> (-2.5%)	:arrow_down:
pkg/expr/func.go	97.34% <0%> (-1.17%)	:arrow_down:
pkg/il/evaluator/evaluator.go	81.67% <0%> (-1.11%)	:arrow_down:
pkg/il/compiler/compiler.go	88.38% <0%> (+0.19%)	:arrow_up:

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