Bigtable: integrate OpenCensus tracing into the bigtable data client

[igorbernstein2]

This adds instrumentation from gax-java to enable tracing of the Bigtable client data operations.

[igorbernstein2]

[codecov]

Codecov Report

Merging #4493 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #4493 +/- ##
============================================
+ Coverage 49.15% 49.19% +0.04% - Complexity 21940 21970 +30 
============================================
Files 2077 2077 Lines 207215 207440 +225 Branches 24108 24107 -1 ============================================
+ Hits 101861 102058 +197 - Misses 97180 97206 +26 - Partials 8174 8176 +2

Impacted Files	Coverage Δ	Complexity Δ
...ble/data/v2/stub/EnhancedBigtableStubSettings.java	98.61% <100%> (+0.04%)	17 <0> (ø)	⬇️
...v2/stub/mutaterows/MutateRowsRetryingCallable.java	92.85% <100%> (ø)	2 <0> (ø)	⬇️
...ud/bigtable/data/v2/stub/EnhancedBigtableStub.java	98.47% <100%> (+0.12%)	22 <5> (+2)	⬆️
...ud/bigtable/gaxx/tracing/WrappedTracerFactory.java	100% <100%> (ø)	2 <2> (?)
.../v2/stub/mutaterows/MutateRowsAttemptCallable.java	89.1% <100%> (+0.33%)	19 <0> (ø)	⬇️
.../com/google/cloud/redis/v1/CloudRedisSettings.java	12.72% <0%> (-3.56%)	2% <0%> (ø)
...com/google/cloud/redis/v1/stub/CloudRedisStub.java	5.88% <0%> (-3.21%)	1% <0%> (ø)
...n/java/com/google/cloud/bigquery/BigQueryImpl.java	83.46% <0%> (-0.81%)	49% <0%> (ø)
...ud/bigtable/admin/v2/BigtableTableAdminClient.java	78.63% <0%> (-0.36%)	27% <0%> (-1%)
...bigtable/admin/v2/BigtableInstanceAdminClient.java	97.09% <0%> (-0.03%)	45% <0%> (-1%)
... and 7 more

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[igorbernstein2]

I'm not sure how to ensure that the opencensus versions in this readme stay up to date with google-cloud-clients/pom.xml properties. I thought about using version markers, but those seem to only be used for internal artifacts. It would be awesome if version.txt could be generalized to manage static external versions as well.

[sduskis]

Kokoro - Test: Code Format failed.

[igorbernstein2]

Format fixed