Infer resource type based on conventions for export to Google Cloud Monitoring (Stackdriver)

[nilebox]

Description:
Reverse mapping from OpenTelemetry's semantic conventions to OpenCensus resource type.

Since OpenTelemetry has switched to using semantic conventions over explicit resource types, the resource type may now be missing when converted to OpenCensus resource.
In that case we will try to infer the resource type based on documented conventions.

Link to tracking Issue:
The issue has been originally discussed in open-telemetry/opentelemetry-python#951 (comment), pointing to inconsistency between OpenCensus and OpenTelemetry which needs to be addressed.

Testing: Unit tests added for new functionality

Documentation: No documentation needed, we may just refer to the already documented semantic conventions.

/cc @james-bebbington @aabmass

[nilebox]

Actually, this feature seems generic and may be useful for other exporters relying on backward compatibility with OpenCensus.
Would it make sense to move it to the core repo (https://github.com/open-telemetry/opentelemetry-collector/blob/master/translator/internaldata/resource_to_oc.go) ?

[codecov]

Codecov Report

Merging #581 into master will decrease coverage by 0.02%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
- Coverage   86.21%   86.19%   -0.03%     
==========================================
  Files         195      196       +1     
  Lines       10608    10618      +10     
==========================================
+ Hits         9146     9152       +6     
- Misses       1129     1132       +3     
- Partials      333      334       +1     

Flag	Coverage Δ
#integration	71.09% <ø> (ø)
#unit	86.02% <60.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exporter/stackdriverexporter/resource_mapper.go	86.95% <0.00%> (-13.05%)	⬇️
...porter/stackdriverexporter/resource_type_mapper.go	85.71% <85.71%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update be70db2...907ccd5. Read the comment docs.

[james-bebbington]

Agree that open-telemetry/opentelemetry-collector#1462 is preferred solution if approved

[nilebox]

Will close this PR for now, and will re-open if the core PR gets rejected.