Initial switch to OpenTelemetry

[hannahhoward]

Goals

Switch the underlying tracing library to OpenTelemetry, with a bridge to OpenCensus for
compatibility

But why? cause OpenCensus is merged into OpenTelemetry and will go away one day in the future.

Implementation

Don't worry, this changes none of your existing OpenCensus calls! But it switches the underlying library to OpenTelemetry, using an Opencensus Bridge.

Theoretically, no changes. However, libraries that use OpenTelemetry might now get their traces picked up (see for example... the recently merged Graphsync OpenTelemetry instrumentation

But bonus -- you can no start converting your OpenCensus traces... at your leisure. Theoretically, all the spans will still stack.

What about metrics? -- there's a bridge for that too, but I haven't attempted to set it up here.

Why is this so complicated and weird? - a great question. I find it ridiculously complicated too and wish I didn't know about all these OpenSomeFakeInstrumentationStandardThatWillChangeAgainInACoupleYears libraries as part of my career as a programmer, but now I've had to learn them just to get the damn tracing working.

[codecov]

Codecov Report

Merging #7725 (bfd1099) into master (0cfc344) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7725      +/-   ##
==========================================
- Coverage   39.59%   39.52%   -0.08%     
==========================================
  Files         640      640              
  Lines       68326    68332       +6     
==========================================
- Hits        27056    27009      -47     
- Misses      36631    36675      +44     
- Partials     4639     4648       +9     

Impacted Files	Coverage Δ
cmd/lotus-miner/main.go	4.23% <0.00%> (-0.04%)	⬇️
cmd/lotus/main.go	0.00% <0.00%> (ø)
lib/tracing/setup.go	0.00% <0.00%> (ø)
extern/sector-storage/worker_tracked.go	76.53% <0.00%> (-8.17%)	⬇️
chain/events/observer.go	71.64% <0.00%> (-6.72%)	⬇️
chain/rand/rand.go	57.89% <0.00%> (-6.32%)	⬇️
markets/retrievaladapter/client_blockstore.go	62.50% <0.00%> (-6.25%)	⬇️
chain/stmgr/call.go	67.87% <0.00%> (-3.64%)	⬇️
chain/types/blockheader.go	55.55% <0.00%> (-2.23%)	⬇️
storage/wdpost_sched.go	75.49% <0.00%> (-1.97%)	⬇️
... and 13 more

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