Mesh summaries do not respect name scopes

[wchargin]

The docs for tensorboard.plugins.mesh.summary_v2.mesh say:

    Args:
      name: A name for this summary. The summary tag used for TensorBoard will
        be this name prefixed by any active name scopes.

But this is not accurate. Consider the following simple repro:

import tensorflow as tf
from tensorboard.plugins.mesh import summary_v2 as mesh_summary

tf.summary.create_file_writer("logs").set_as_default()

with tf.name_scope("foo"):
    with tf.name_scope("bar"):
        tf.summary.scalar("loss", 0.125, step=0)
        mesh_summary.mesh(
            "mymesh", [[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]]], step=1
        )

This creates a properly scoped scalar summary, foo/bar/loss:

But the mesh summary that it creates is not scoped:

And indeed the written data does not contain the scoped mesh tag name:

$ strings logs/*
brain.Event:2U
foo/bar/lossB
scalars@
mymesh_VERTEXB*
mesh
mymesh

This appears to be because the mesh summary code calls summary_scope
and enters its context manager but discards the yielded tag name:

tensorboard/tensorboard/plugins/mesh/summary_v2.py

Lines 136 to 141 in 1780833

	summary_scope = (
	getattr(tf.summary.experimental, "summary_scope", None)
	or tf.summary.summary_scope
	)
	all_success = True
	with summary_scope(name, "mesh_summary", values=tensors):

[wchargin]

cc @podlipensky

[wchargin]

(Repro is at tb-nightly==2.2.0a20200106.)