add oceanbase namespace for observer

[nautaa]

Enhancement
currently we have add oceanbase namespace for src/oblsm. we also need to add namespace for src/observer, for example:

namespace oceanbase {

class Foo {};
}

ref: https://en.cppreference.com/w/cpp/language/namespace

[aradhanahegde]

could you please assign me this issue

[hnwyllmm]

could you please assign me this issue

assigned, thanks.