prometheus exporter: remove non-ucum units from conversion

[dashpole]

Description:

Discovered during open-telemetry/opentelemetry-go#4374 (review).

B means bel in UCUM, so MB is megabel, for example. $ is also not UCUM.

https://ucum.org/ucum#section-Alphabetic-Index-By-Symbol

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (c0f0ff8) 91.25% compared to head (0a9cc5b) 91.24%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5719      +/-   ##
============================================
- Coverage     91.25%   91.24%   -0.01%     
+ Complexity     5051     5045       -6     
============================================
  Files           558      558              
  Lines         14930    14924       -6     
  Branches       1397     1397              
============================================
- Hits          13624    13618       -6     
  Misses          899      899              
  Partials        407      407              

Files Changed	Coverage Δ
...try/exporter/prometheus/PrometheusUnitsHelper.java	100.00% <ø> (ø)

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[dashpole]

Failure seems unrelated:

> Task :sdk:metrics:compileTestJava
/home/runner/work/opentelemetry-java/opentelemetry-java/sdk/metrics/src/test/java/io/opentelemetry/sdk/metrics/internal/descriptor/MetricDescriptorTest.java:86: error: cannot find symbol
                Advice.create(Arrays.asList(1.0, 2.0))));
                      ^
  symbol:   method create(List<Double>)
  location: class Advice
1 error

> Task :sdk:metrics:compileTestJava FAILED

[jack-berg]

Failure seems unrelated:

Yup, bad merge. Fixed in #5721. Pull in main after that is merged.