Add missing logging for metrics at 'debug' level

[nilebox]

Description:
Fixing a bug - enable metrics' debug logging in Logging Exporter

Link to tracking Issue: Fixes #1107

Testing:
Manual testing: simply run a collector with --log-level=DEBUG and the following config:

receivers:
  prometheus:
    config:
      scrape_configs:
      - job_name: "test"
        scrape_interval: 60s
        static_configs:
        - targets: ["localhost:8888"]
exporters:
  logging:
    loglevel: debug
service:
  pipelines:
    metrics:
      receivers: [prometheus]
      exporters: [logging]

The log now includes detailed information at the DEBUG level:

2020-06-11T14:13:08.697+1000	INFO	loggingexporter/logging_exporter.go:169	MetricsExporter	{"#metrics": 4}
2020-06-11T14:13:08.697+1000	DEBUG	loggingexporter/logging_exporter.go:213	ResourceMetrics #0
Resource labels:
     -> port: STRING(8888)
     -> scheme: STRING(http)
     -> service.name: STRING(test)
     -> host.hostname: STRING(localhost)
InstrumentationLibraryMetrics #0
Metric #0
Descriptor:
     -> Name: otelcol_process_cpu_seconds
     -> Description: Total CPU user and system time in seconds
     -> Unit: s
     -> Type: GAUGE_DOUBLE
Metric #1
Descriptor:
     -> Name: otelcol_process_runtime_heap_alloc_bytes
     -> Description: Bytes of allocated heap objects (see 'go doc runtime.MemStats.HeapAlloc')
     -> Unit: By
     -> Type: GAUGE_DOUBLE
Metric #2
Descriptor:
     -> Name: otelcol_process_runtime_total_alloc_bytes
     -> Description: Cumulative bytes allocated for heap objects (see 'go doc runtime.MemStats.TotalAlloc')
     -> Unit: By
     -> Type: GAUGE_DOUBLE
Metric #3
Descriptor:
     -> Name: otelcol_process_runtime_total_sys_memory_bytes
     -> Description: Total bytes of memory obtained from the OS (see 'go doc runtime.MemStats.Sys')
     -> Unit: By
     -> Type: GAUGE_DOUBLE

Documentation: N/A

[codecov]

Codecov Report

Merging #1108 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1108   +/-   ##
=======================================
  Coverage   86.45%   86.45%           
=======================================
  Files         198      198           
  Lines       14171    14172    +1     
=======================================
+ Hits        12251    12252    +1     
  Misses       1468     1468           
  Partials      452      452           

Impacted Files	Coverage Δ
exporter/loggingexporter/logging_exporter.go	80.14% <100.00%> (+0.14%)	⬆️
translator/internaldata/resource_to_oc.go	76.47% <0.00%> (ø)

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[dmitryax]

LGTM