xds: add support for setting bootstrap file with java system property

[erikjoh]

While most languages support setting environment variables during runtime,
Java does not. In Java, the preferred approach is to use Java System Properties
in order so specify configuration options. By checking for the existence
of the io.grpc.xds.bootstrap property if GRPC_XDS_BOOTSTRAP is not found,
it is possible to either supply the bootstrap location during runtime or as
a Java argument. The environment variable still takes precedence in order
to not break any existing documentation.

For discussion see #7605 .

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Erik Johansson (6ff16a0)

[linux-foundation-easycla]

• ❌ Erik Johansson The commit (f5065b5) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[erikjoh]

Blocked waiting for Corporate CLA approval.

[linux-foundation-easycla]

• ❌ Erik Johansson The commit (f5065b5) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[erikjoh]

@ejona86 @sanjaypujare FYI the link here https://github.com/grpc/grpc-java/blob/master/CONTRIBUTING.md#legal-requirements (https://identity.linuxfoundation.org/projects/cncf) goes to the old and now outdated system.
We finally figured out how to get me signed though so now this PR is ready for review 🙂

[ejona86]

It seems like maybe we don't need to mention the system property in the example, but we don't actually have another place to put that documentation so it seems best for now.

[sanjaypujare]

Added 2 comments but looks good.

[sanjaypujare]

@erikjoh it is better if you don't squash commits and "force-push" them - that way it is easier to see the changes made in the second commit. While merging the commits can be squashed.

[sanjaypujare]

LG

[erikjoh]

@erikjoh it is better if you don't squash commits and "force-push" them - that way it is easier to see the changes made in the second commit. While merging the commits can be squashed.

Ah sorry, I don't use the squash merge feature a lot! Noted for the next time 🙂
You'll have to merge it once the build finishes since I don't have permission to do so.
Thanks for taking the time to have a look at this!

[erikjoh]

@sanjaypujare That build failure seems unrelated to this since the class was introduced recently and does not exist on my branch even. Doesn't seem like I have any option to rerun the build.

io.grpc.xds.SharedCallCounterMapTest > autoCleanUp FAILED
    expected to be empty
    but was: {cluster-foo.googleapis.com={null=io.grpc.xds.SharedCallCounterMap$CounterReference@21c8125f}}
        at io.grpc.xds.SharedCallCounterMapTest.autoCleanUp(SharedCallCounterMapTest.java:57)

[ejona86]

Thank you!