processor_content_modifier: add support for Metrics processing

[edsiper]

The following PR extends Content Modifier processor allowing to operate on metrics generated originally by OpenTelemetry. The following contexts are available for metrics:

• otel_resource_attributes
• otel_scope_name
• otel_scope_version

usage example:

service:
  log_level: info

pipeline:
  inputs:
    - name: opentelemetry
      processors:
        metrics:
          - name: content_modifier
            context: otel_scope_name
            action: upsert
            value: "fluent-bit-scope"

          - name: content_modifier
            context: otel_scope_attributes
            action: upsert
            key: scope_attr_processor
            value: "attributes fluent-bit"

          - name: content_modifier
            context: otel_scope_name
            action: upsert
            value: "new-scope-name"

          - name: content_modifier
            context: otel_resource_attributes
            action: insert
            key: "added_by"
            value: "fluent-bit"

          - name: content_modifier
            context: otel_resource_attributes
            action: insert
            key: "http.url"
            value: https://fluentbit.io/blog/?this=that

          - name: content_modifier
            context: otel_resource_attributes
            action: upsert
            key: "added_by"
            value: "fluent-bit v3.2"

          - name: content_modifier
            context: otel_resource_attributes
            action: delete
            key: "service.name"

          - name: content_modifier
            context: otel_resource_attributes
            action: rename
            key: "telemetry.sdk.language"
            value: "my-sdk"

          - name: content_modifier
            context: otel_resource_attributes
            action: hash
            key: "host.hostname"

          - name: content_modifier
            context: otel_resource_attributes
            action: extract
            key: "http.url"
            pattern: ^(?<http_protocol>https?):\/\/(?<http_domain>[^\/\?]+)(?<http_path>\/[^?]*)?(?:\?(?<http_query_params>.*))?

          - name: content_modifier
            context: otel_resource_attributes
            action: insert
            key: "is_it_sunny"
            value: "true"

          - name: content_modifier
            context: otel_resource_attributes
            action: convert
            key: is_it_sunny
            converted_type: boolean

  outputs:
    - name: stdout
      match: "*"

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.