[Observability] feat: add metrics of http response

[TedaLIEz]

Summary

This pull request enhances the OpenTelemetry instrumentation in the ext_otel.py file by introducing a new HTTP response counter to track the total number of HTTP responses by status code. It also updates the imports to include the required get_meter function for metrics.

Enhancements to OpenTelemetry instrumentation:

• Added HTTP response counter for metrics tracking: Introduced a new counter _http_response_counter using the get_meter function to track the total number of HTTP responses, categorized by status code and status class. This counter is updated in the response_hook function. ([api/extensions/ext_otel.pyR115-R131](https://github.com/langgenius/dify/pull/18499/files#diff-d9b99e6e386de2f18f686daf1e6fcb87ca02c20900826cbdc5892a5e81f362b6R115-R131))

• Updated imports for metrics: Added the get_meter import from opentelemetry.metrics to support the new HTTP response counter functionality. ([api/extensions/ext_otel.pyL17-R17](https://github.com/langgenius/dify/pull/18499/files#diff-d9b99e6e386de2f18f686daf1e6fcb87ca02c20900826cbdc5892a5e81f362b6L17-R17))

We can use sum(increase(http_server_response_count{status_class=~"4xx|5xx"}[30m])) / sum(increase(http_server_response_count[30m])) * 100 to measure the error rate after setting up this metric.

Screenshots

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods