feat: introducing bulk read API through Batcher

[rahulKQL]

Fixes #7

This change introduces BulkReadAPI on BigtableDataClient. This operation
accepts row keys in a batch mode and behind the scene split them based
on configurable values.

[codecov]

Codecov Report

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The diff coverage is 93.68%.

@@ Coverage Diff @@ ## master #99 +/- ## ========================================= Coverage ? 81.92% Complexity ? 971 ========================================= Files ? 99 Lines ? 6021 Branches ? 330 ========================================= Hits ? 4933 Misses ? 910 Partials ? 178

Impacted Files	Coverage Δ	Complexity Δ
...ud/bigtable/data/v2/stub/EnhancedBigtableStub.java	94.4% <0%> (ø)	19 <0> (?)
...data/v2/stub/BigtableBulkReadRowsCallSettings.java	100% <100%> (ø)	5 <5> (?)
...a/v2/stub/readrows/ReadRowsBatchingDescriptor.java	100% <100%> (ø)	8 <8> (?)
...gle/cloud/bigtable/data/v2/BigtableDataClient.java	92.3% <100%> (ø)	35 <3> (?)
...om/google/cloud/bigtable/data/v2/models/Query.java	67.61% <100%> (ø)	26 <1> (?)
...ble/data/v2/stub/EnhancedBigtableStubSettings.java	95.02% <100%> (ø)	20 <2> (?)

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[igorbernstein2]

What are you testing here?

[rahulKQL]

Thanks for pointing it out, Have updated this with valid test case.

[igorbernstein2]

todo should move to getDefaultChannelPoolSize

[igorbernstein2]

LGTM
@kolea2 do you want to take a pass as well?

[kolea2]

can we make this a variable?

[rahulKQL]

We could have a variable here. Although as in this format, it is similar to the existing bulkMutateRowsSettings configuration

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/EnhancedBigtableStubSettings.java

Lines 500 to 516 in abbc6c8

	bulkMutateRowsSettings =
	BigtableBatchingCallSettings.newBuilder(new MutateRowsBatchingDescriptor())
	.setRetryableCodes(IDEMPOTENT_RETRY_CODES)
	.setRetrySettings(MUTATE_ROWS_RETRY_SETTINGS)
	.setBatchingSettings(
	BatchingSettings.newBuilder()
	.setIsEnabled(true)
	.setElementCountThreshold(100L)
	.setRequestByteThreshold(20L * 1024 * 1024)
	.setDelayThreshold(Duration.ofSeconds(1))
	.setFlowControlSettings(
	FlowControlSettings.newBuilder()
	.setLimitExceededBehavior(LimitExceededBehavior.Block)
	.setMaxOutstandingRequestBytes(100L * 1024 * 1024)
	.setMaxOutstandingElementCount(1_000L)
	.build())
	.build());

We might need to change that to have common formatting in separate PR

[kolea2]

Added a few small comments but LGTM overall!