\version "2.19.10"
\language "english"

\new Staff {
    \once \override TextSpanner.bound-details.left.attach-dir = #-7
    \once \override TextSpanner.arrow-width = 0.25
    \once \override TextSpanner.bound-details.left.text = \markup { 
        \smaller
            \general-align
                #Y
                #DOWN
                \note-by-number
                    #2
                    #0
                    #1
        \upright
            " = 67.5"
        }
    \once \override TextSpanner.bound-details.right.arrow = ##t
    \once \override TextSpanner.bound-details.right.padding = 2
    c'2 \startTextSpan
    d'2
    e'2
    f'2
    g'2
    a'2 \stopTextSpan ^ \markup {
        \smaller
            \general-align
                #Y
                #DOWN
                \note-by-number
                    #2
                    #0
                    #1
        \upright
            " = 135"
        }
}

%%% or in short :

{ 
  \once \override TextSpanner.arrow-width = 0.25
  \once \override TextSpanner.bound-details =
              #`((left
                  (text . ,#{ \markup {
                    \smaller
                    \general-align #Y #DOWN
                    \note-by-number #2 #0 #1
                    \upright
                    " = 67.5"
                  } #})
                  (Y . 0)
                  (padding . 0.25)
                  (attach-dir . -7))
                 (right
                  (arrow . #t)
                  (text . ,#{ \markup { " "
                    \smaller
                    \general-align #Y #DOWN
                    \note-by-number #2 #0 #1
                    \upright
                    " = 135"
                  } #})
                  (Y . 0)
                  (padding . 2)
                  (attach-dir . 0)))
  c'2 \startTextSpan
  d'2
  e'2
  f'2
  g'2
  a'2 \stopTextSpan  
}