enh: Add formatter to Server HoverTool

[hoxbro]

import datetime as dt

import datashader as ds
import numpy as np
import pandas as pd
import panel as pn

import holoviews as hv
from holoviews.operation.datashader import rasterize

hv.extension("bokeh")

num = 10000
seed = np.random.default_rng(1)


dists = {
    cat: pd.DataFrame(
        {
            "x": seed.normal(x, s, num),
            "y": seed.normal(y, s, num),
            "s": s,
            "val": val,
            "cat": cat,
            "dt": dt.datetime(2020, 1, val + 1),
        }
    )
    for x, y, s, val, cat in [
        (2, 2, 0.03, 0, "d1"),
        (2, -2, 0.10, 1, "d2"),
        (-2, -2, 0.50, 2, "d3"),
        (-2, 2, 1.00, 3, "d4"),
        (0, 0, 3.00, 4, "d5"),
    ]
}

df = pd.concat(dists, ignore_index=True)
df["y"] += 100  # Offset to distinguish x from y
points = hv.Points(df)

plot = rasterize(points, aggregator=ds.min("s"), selector=ds.min("s")).opts(tools=["hover"])
pn.panel(plot).servable()

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.81%. Comparing base (99d3528) to head (750d953).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/plotting/bokeh/raster.py	73.33%	4 Missing ⚠️
holoviews/tests/ui/bokeh/test_hover.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6531      +/-   ##
==========================================
+ Coverage   88.79%   88.81%   +0.01%     
==========================================
  Files         323      323              
  Lines       68960    68988      +28     
==========================================
+ Hits        61236    61270      +34     
+ Misses       7724     7718       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[hoxbro]

Opened an issue in Bokeh: bokeh/bokeh#14413

[hoxbro]

We should merge this PR as is, as the datetime fix will likely sort itself with an upstream fix.

[hoxbro]

It would be OK with me to only have the BOKEH_GE_3_7_0 code.

[philippjfr]

What's the behavior for older bokeh versions?

[hoxbro]

This is only for the x,y-coords, so it would show 1.999999999999 instead of 2.0.

Though, I'm unsure how it would work if x,y-coords are datetime.