Use minimal fallback managed channel when none is specified

[jack-berg]

Fixes #5958.

See comment describing need for this change:

• If you include opentelemetry-exporter-sender-grpc-managed-channel and exclude opentelemetry-exporter-sender-okhttp, autoconfigure will fail to initialize because opentelemetry-exporter-sender-grpc-managed-channel expects the user to call setChannel, and there's no chance to do that before the span exporter customizer gets called, so we get a NPE.
• The NPE occurs because UpstreamGrpcSenderProvider doesn't have any way to accommodate a null managedChannel argument.
• I'm pushing a fix in Use minimal fallback managed channel when none is specified #6110 which updates UpstreamGrpcSenderProvider to initialize a minimal managed channel is none is explicitly set by the user. This will allow autoconfigure to initialize without error, but still requires the user to use a span exporter customer to customize any of the options of the managed channel.

[codecov]

Codecov Report

Attention: 88 lines in your changes are missing coverage. Please review.

Comparison is base (de65a4b) 91.12% compared to head (42cd12e) 90.99%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
.../exporter/prometheus/Otel2PrometheusConverter.java	71.53%	58 Missing and 20 partials ⚠️
...porter/prometheus/PrometheusHttpServerBuilder.java	37.50%	5 Missing ⚠️
...etry/exporter/prometheus/PrometheusHttpServer.java	89.65%	3 Missing ⚠️
...ry/exporter/prometheus/PrometheusMetricReader.java	90.00%	1 Missing ⚠️
...edchannel/internal/UpstreamGrpcSenderProvider.java	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6110      +/-   ##
============================================
- Coverage     91.12%   90.99%   -0.13%     
+ Complexity     5742     5690      -52     
============================================
  Files           630      628       -2     
  Lines         16844    16688     -156     
  Branches       1664     1656       -8     
============================================
- Hits          15349    15186     -163     
- Misses         1029     1047      +18     
+ Partials        466      455      -11     

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