feat: add opencensus tracing support

[athakor]

Fixes #359

[codecov]

Codecov Report

Merging #360 into master will decrease coverage by 0.18%.
The diff coverage is 41.46%.

@@             Coverage Diff              @@
##             master     #360      +/-   ##
============================================
- Coverage     72.64%   72.45%   -0.19%     
+ Complexity     1056     1048       -8     
============================================
  Files            64       64              
  Lines          5535     5570      +35     
  Branches        684      686       +2     
============================================
+ Hits           4021     4036      +15     
- Misses         1301     1320      +19     
- Partials        213      214       +1     

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/cloud/firestore/CollectionGroup.java	20.58% <0.00%> (-2.75%)	1.00 <0.00> (ø)
...om/google/cloud/firestore/CollectionReference.java	43.75% <0.00%> (-3.98%)	10.00 <0.00> (ø)
.../com/google/cloud/firestore/DocumentReference.java	70.32% <0.00%> (-3.24%)	31.00 <0.00> (ø)
...rc/main/java/com/google/cloud/firestore/Query.java	87.93% <ø> (ø)	120.00 <0.00> (-7.00)
...java/com/google/cloud/firestore/FirestoreImpl.java	76.86% <50.00%> (-1.71%)	26.00 <0.00> (ø)
...ain/java/com/google/cloud/firestore/TraceUtil.java	50.00% <57.14%> (+25.00%)	3.00 <2.00> (+2.00)
...n/java/com/google/cloud/firestore/Transaction.java	86.27% <100.00%> (+0.85%)	11.00 <3.00> (ø)
...java/com/google/cloud/firestore/UpdateBuilder.java	95.63% <100.00%> (+0.04%)	83.00 <0.00> (ø)
...rc/main/java/com/google/cloud/firestore/Watch.java	91.70% <100.00%> (+0.03%)	56.00 <0.00> (ø)
...in/java/com/google/cloud/firestore/BulkWriter.java	71.02% <0.00%> (-0.47%)	28.00% <0.00%> (-1.00%)

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