Self-instrumentation options; event timestamp bug fix

[jmacd]

Description:

1. Renames the SDK's otelcol exporter name to indicate whether traces or metrics
2. Fixes a span event timestamp bug (use of Nanoseconds vs UnixNano,🩸)
3. Adds boolean options for self-spans and self-metrics
4. Disables self-spans by default, enables self-metrics by default

[codecov-commenter]

Codecov Report

Patch coverage: 55.55% and project coverage change: -0.23 ⚠️

Comparison is base (a56cd59) 89.72% compared to head (5c3e341) 89.49%.

❗ Current head 5c3e341 differs from pull request most recent head 32335c5. Consider uploading reports for the commit 32335c5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #494      +/-   ##
==========================================
- Coverage   89.72%   89.49%   -0.23%     
==========================================
  Files          62       62              
  Lines        3552     3570      +18     
==========================================
+ Hits         3187     3195       +8     
- Misses        277      281       +4     
- Partials       88       94       +6     

Impacted Files	Coverage Δ
lightstep/sdk/metric/internal/doevery/doevery.go	76.19% <ø> (ø)
...p/sdk/metric/internal/syncstate/refcount_mapped.go	100.00% <ø> (ø)
...htstep/sdk/metric/exporters/otlp/otelcol/client.go	71.92% <53.84%> (-2.60%)	⬇️
...ghtstep/sdk/trace/exporters/otlp/otelcol/client.go	66.66% <53.84%> (-1.71%)	⬇️
lightstep/sdk/trace/exporters/otlp/otelcol/otlp.go	81.25% <100.00%> (ø)

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