Update default host and port for PrometheusExporter to align with the OTel spec.

[Yun-Ting]

Fixes #.
#2780

Changes

Update default httpListenerPrefixes for PrometheusExporter to be http://localhost:9464/ to align with the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#prometheus-exporter

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

• Appropriate CHANGELOG.md updated for non-trivial changes
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2783 (5926191) into main (1150f4a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2783   +/-   ##
=======================================
  Coverage   83.75%   83.75%           
=======================================
  Files         251      251           
  Lines        8864     8864           
=======================================
  Hits         7424     7424           
  Misses       1440     1440           

Impacted Files	Coverage Δ
...y.Exporter.Prometheus/PrometheusExporterOptions.cs	84.21% <100.00%> (ø)

[reyang]

Couple things to discuss:

1. Do we want the binding host/port to be optional or required? (e.g. it seems Java implementation is making it required https://github.com/open-telemetry/opentelemetry-java/tree/main/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus, prom-client for Python makes the port a required thing https://github.com/prometheus/client_python/blob/038b5bd9357929a1e4e0cfa16c0c407a66702c0b/prometheus_client/exposition.py#L126-L135)
2. If the answer to 1) is optional, what should be the default value and why?
3. Given the .NET PrometheusExporter can be used either as a standalone HttpListener or integrated into ASP.NET Core pipeline, do we want to have different default value depending on the usage case, or the same value across the board? (@CodeBlanch you might have more insights here).

Ah apologies, didn't see the open questions. Something made me think the default had already been decided.

[alanwest]

it seems Java implementation is making it required

Looks like Java's default is 9464

Appears there is some precedent for 9464 as here's the OpenCensus exporter for node https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-exporter-prometheus/README.md.

The spec references it too https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#prometheus-exporter

[Yun-Ting]

it seems Java implementation is making it required

Looks like Java's default is 9464

Appears there is some precedent for 9464 as here's the OpenCensus exporter for node https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-exporter-prometheus/README.md.

The spec references it too https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#prometheus-exporter

Interesting! Not sure how did people come up with 9464. BTW, here contains the list of ports that we may need to avoid when choosing default ports.