Add Trace::Status.from_http_status

* based on google apis docs:
  https://cloud.google.com/apis/design/errors#handling_errors

There's also https://github.com/grpc/grpc/blob/master/doc/http-grpc-status-mapping.md
which mentions:
"Servers must not use this table to determine an HTTP status
code to use; the mappings are neither symmetric nor 1-to-1.", so...

api/lib/opentelemetry/trace/status.rb

@@ -19,9 +19,56 @@ class Status
       # @return [String]
       attr_reader :description
 
+      # Implemented according to
+      # https://cloud.google.com/apis/design/errors#handling_errors
+      #
+      # Note that some HTTP status do not map 1-to-1 to a gRPC status.
+      #
+      # @param code Numeric HTTP status
+      #
+      # @return Status
+      def self.from_http_status(code) # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity, Metrics/MethodLength
+        case code.to_i
+        when 200
+          new(OK)
+        when 400
+          new(INVALID_ARGUMENT)
+          # or, possibly (no one-to-one mapping):
+          # new(FAILED_PRECONDITION)
+          # new(OUT_OF_RANGE)
+        when 401
+          new(UNAUTHENTICATED)
+        when 403
+          new(PERMISSION_DENIED)
+        when 404
+          new(NOT_FOUND)
+        when 409
+          new(ABORTED)
+          # or, possibly (no one-to-one mapping):
+          # new(ALREADY_EXISTS)
+        when 429
+          new(RESOURCE_EXHAUSTED)
+        when 499
+          new(CANCELLED)
+        when 500
+          new(DATA_LOSS)
+          # or, possibly (no one-to-one mapping):
+          # new(UNKNOWN_ERROR)
+          # new(INTERNAL_ERROR)
+        when 501
+          new(UNIMPLEMENTED)
+        when 503
+          new(UNAVAILABLE)
+        when 504
+          new(DEADLINE_EXCEEDED)
+        else
+          new(UNKNOWN_ERROR)
+        end
+      end
+
       # Initialize a Status.
       #
-      # @param [Integer] canonical_code One of the standard GRPC codes: https://github.com/grpc/grpc/blob/master/doc/statuscodes.md
+      # @param [Integer] canonical_code One of the standard gRPC codes: https://github.com/grpc/grpc/blob/master/doc/statuscodes.md
       # @param [String] description
       def initialize(canonical_code, description: '')
         @canonical_code = canonical_code
@@ -36,7 +83,7 @@ def ok?
       end
 
       # The following represents the canonical set of status codes of a
-      # finished {Span}, following the standard GRPC codes:
+      # finished {Span}, following the standard gRPC codes:
       # https://github.com/grpc/grpc/blob/master/doc/statuscodes.md
 
       # The operation completed successfully.

api/test/opentelemetry/trace/status_test.rb

@@ -7,6 +7,40 @@
 require 'test_helper'
 
 describe OpenTelemetry::Trace::Status do
+  let(:trace_status) { OpenTelemetry::Trace::Status }
+
+  describe '.from_http_status' do
+    it 'returns Status' do
+      _(trace_status.from_http_status(400)).must_be_kind_of trace_status
+    end
+
+    def assert_http_to_status(http_code, trace_status_code)
+      _(trace_status.from_http_status(http_code).canonical_code).must_equal trace_status_code
+    end
+
+    it 'maps http 200' do
+      assert_http_to_status(200, trace_status::OK)
+    end
+
+    it 'maps common 4xx http codes' do
+      assert_http_to_status(400, trace_status::INVALID_ARGUMENT)
+      assert_http_to_status(401, trace_status::UNAUTHENTICATED)
+      assert_http_to_status(403, trace_status::PERMISSION_DENIED)
+      assert_http_to_status(404, trace_status::NOT_FOUND)
+      assert_http_to_status(409, trace_status::ABORTED)
+      assert_http_to_status(429, trace_status::RESOURCE_EXHAUSTED)
+      assert_http_to_status(499, trace_status::CANCELLED)
+    end
+
+    it 'maps common 5xx http codes' do
+      assert_http_to_status(500, trace_status::DATA_LOSS)
+      assert_http_to_status(501, trace_status::UNIMPLEMENTED)
+      assert_http_to_status(502, trace_status::UNKNOWN_ERROR)
+      assert_http_to_status(503, trace_status::UNAVAILABLE)
+      assert_http_to_status(504, trace_status::DEADLINE_EXCEEDED)
+    end
+  end
+
   describe '.canonical_code' do
     it 'reflects the value passed in' do
       status = OpenTelemetry::Trace::Status.new(0)