Fix InstantiatingGrpcChannelProvider's channel pool to play nicely with DirectPath

[igorbernstein2]

// cc @WeiranFang

By default grpclb strategy for DirectPath is to create a subchannel for
every address resolved by the grpclb and round robin over them.
Unfortunately this doesn't work well when Channel pooling is enable in
the InstantiatingGrpcChannelProvider, which will create multiple
ManagedChannels, each containing a bunch of subchannels.

Since channel pooling is needed to have good performance targeting CFEs,
the solution here is to force each ManagedChannel to pick a single
subchannel. Thus preserving the CFE behavior of a single ManagedChannel
only containing a single subchannel.

Unfortunately I couldn't figure out a reasonable way to write tests for this. ManagedChannelBuilder nor ManagedChannel provide a way from me to introspect what the current service config is, so a unit test w/o reflection is not possible. And bringing up a direct path like environment for a functional test is a bit too much.

[codecov]

Codecov Report

Merging #798 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #798      +/-   ##
============================================
+ Coverage     78.39%   78.43%   +0.04%     
  Complexity     1106     1106              
============================================
  Files           198      198              
  Lines          4887     4897      +10     
  Branches        385      385              
============================================
+ Hits           3831     3841      +10     
  Misses          887      887              
  Partials        169      169

Impacted Files	Coverage Δ	Complexity Δ
...api/gax/grpc/InstantiatingGrpcChannelProvider.java	79.44% <100%> (+1.2%)	36 <0> (ø)	⬇️

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[chingor13]

Is there a link to the documentation somewhere about the configuration we're specifying? I couldn't find much on the ForwardingChannelBuilder docs. The map of list of maps looks pretty arbitrary at first glance.

[igorbernstein2]

Maybe @WeiranFang can comment on the service config semantics? I don't know of any docs for the service config format. The config is the translation of the serverside config:
{"loadBalancingConfig":[{"grpclb":{"childPolicy":[{"pick_first":{}}]}}]}

See cl/243322471, for the original source. I reformatted the code and introduced local variables to make it easier to read. But unfortunately I can't offer more info on the semantics

[WeiranFang]

Hi @chingor13 , the usage of this defaultServiceConfig is defined in ManagedChannelBuilder. And the implementation is in AbstractManagedChannelImplBuilder.

I also found the spec for adding this API.

As for the semantics of service config, check the GrpcLbConfig defined in service_config.proto

Thanks!

[igorbernstein2]

@chingor13 PTAL