Improve tracing and logging for write errors.

[hexedpackets]

This adds error flags and exception messages to the spans for
metadata, suggest, and bigtable writes when the chain of futures
fails. The same exceptions are also logged. Previously some
exceptions, such as grpc errors, could be masked by sl4j settings.

The trace for a metric write was cleaned up to remove several
intermediary spans. These spans did not have useful information and
had no branching paths, so they were just clutter in the overall
trace.

Closes #724.

[codecov]

Codecov Report

Merging #740 (5f30b9a) into master (037a871) will increase coverage by 0.20%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #740      +/-   ##
============================================
+ Coverage     54.82%   55.02%   +0.20%     
- Complexity     3105     3130      +25     
============================================
  Files           742      743       +1     
  Lines         20191    20248      +57     
  Branches       1313     1316       +3     
============================================
+ Hits          11070    11142      +72     
+ Misses         8653     8633      -20     
- Partials        468      473       +5     

Impacted Files	Coverage Δ	Complexity Δ
...spotify/heroic/consumer/pubsub/PubSubConsumer.java	50.00% <ø> (ø)	2.00 <0.00> (ø)
...va/com/spotify/heroic/metric/MetricCollection.java	66.66% <ø> (+3.50%)	12.00 <0.00> (ø)
.../com/spotify/heroic/metric/LocalMetricManager.java	64.58% <0.00%> (-1.49%)	4.00 <0.00> (ø)
...ic/metric/bigtable/api/BigtableDataClientImpl.java	20.77% <0.00%> (ø)	4.00 <0.00> (ø)
...istics/semantic/SemanticMetricBackendReporter.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...om/spotify/heroic/consumer/schemas/Spotify100.java	51.78% <37.14%> (+0.89%)	4.00 <0.00> (ø)
...roic/metadata/elasticsearch/MetadataBackendKV.java	80.66% <42.85%> (-1.08%)	50.00 <1.00> (ø)
...heroic/suggest/elasticsearch/SuggestBackendKV.java	81.48% <50.00%> (-0.77%)	77.00 <1.00> (ø)
...potify/heroic/metric/bigtable/BigtableBackend.java	85.81% <59.37%> (-3.12%)	61.00 <3.00> (ø)
...fy/heroic/aggregation/ComputeDistributionStat.java	64.28% <64.28%> (ø)	1.00 <1.00> (?)
... and 20 more

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[ao2017]

LGTM

[ao2017]

Why the replacement?

[hexedpackets]

Just doing some cleanup as I went through the code. While I generally like Optional, in this case its only purpose is a single assume statement in the test, so removing it makes things a tiny bit easier to read/less verbose.