Fix Prometheus server crash on listening to already used port

[lalitb]

Fixes #1903

Changes

Please provide a brief description of the changes here.

• Handle exception thrown by CivetServer (used by prometheus-cpp) if the port to listen is already used.
• Update documentation for Prometheus example (remove reference of PeriodicPushMetricReader) as per prometheus design changes in Convert Prometheus Exporter to Pull MetricReader #1953.
• Change default Prometheus listening port to 9464 in example.

It's difficult to add unit-test to simulate port usage. So tested changes as below:

Step 1: Start a http server at particular port (say 8000):

$python3 -m http.server --cgi 8000
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...

Step 2: Try running Prometheus example to invoke Prometheus exporter on this used port 8000:

$ ./prometheus_example all localhost:8000
PrometheusExporter example program running ...
[Error] File: /<path>/opentelemetry-cpp/exporters/prometheus/src/exporter.cc:26 [Prometheus Exporter] Can't initialize prometheus exposer with endpoint: localhost:8000
Error: null context when constructing CivetServer. Possible problem binding to port.
...
[Warning] File: /<path>/opentelemetry-cpp/sdk/src/metrics/metric_reader.cc:47 MetricReader::Shutdown - Cannot invoke shutdown twice!
$

This fails gracefully.

Step 3: Try running Prometheus example to invoke Prometheus exporter on some unused port (say 8001):

$ ./prometheus_example all localhost:8001
PrometheusExporter example program running ...
...
$

This is successful.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1986 (074ce2d) into main (e47dec5) will increase coverage by 0.03%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1986      +/-   ##
==========================================
+ Coverage   87.11%   87.13%   +0.03%     
==========================================
  Files         166      166              
  Lines        4597     4597              
==========================================
+ Hits         4004     4005       +1     
+ Misses        593      592       -1     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	91.48% <0.00%> (+0.78%)	⬆️

[marcalff]

LGTM, setting the exporter to shutdown looks like the best solution.

[esigo]

LGTM
Thanks for the fix :)